diff --git a/packages/babel-types/scripts/generators/docs.js b/packages/babel-types/scripts/generators/docs.js
index 3bbb52362400..340dbbc11315 100644
--- a/packages/babel-types/scripts/generators/docs.js
+++ b/packages/babel-types/scripts/generators/docs.js
@@ -101,8 +101,12 @@ Object.keys(types.BUILDER_KEYS)
         }
         if (defaultValue !== null || types.NODE_FIELDS[key][field].optional) {
           fieldDescription.push(
-            " (default: `" + util.inspect(defaultValue) + "`)"
+            " (default: `" + util.inspect(defaultValue) + "`"
           );
+          if (types.BUILDER_KEYS[key].indexOf(field) < 0) {
+            fieldDescription.push(", excluded from builder function");
+          }
+          fieldDescription.push(")");
         } else {
           fieldDescription.push(" (required)");
         }
diff --git a/packages/babel-types/src/asserts/generated/index.js b/packages/babel-types/src/asserts/generated/index.js
index 4fb0fd754a15..9f2ebce70e4a 100644
--- a/packages/babel-types/src/asserts/generated/index.js
+++ b/packages/babel-types/src/asserts/generated/index.js
@@ -224,12 +224,12 @@ export function assertArrowFunctionExpression(
 export function assertClassBody(node: Object, opts?: Object = {}): void {
   assert("ClassBody", node, opts);
 }
-export function assertClassDeclaration(node: Object, opts?: Object = {}): void {
-  assert("ClassDeclaration", node, opts);
-}
 export function assertClassExpression(node: Object, opts?: Object = {}): void {
   assert("ClassExpression", node, opts);
 }
+export function assertClassDeclaration(node: Object, opts?: Object = {}): void {
+  assert("ClassDeclaration", node, opts);
+}
 export function assertExportAllDeclaration(
   node: Object,
   opts?: Object = {},
diff --git a/packages/babel-types/src/builders/generated/index.js b/packages/babel-types/src/builders/generated/index.js
index fe7f320d028a..8aebb3a8d830 100644
--- a/packages/babel-types/src/builders/generated/index.js
+++ b/packages/babel-types/src/builders/generated/index.js
@@ -225,14 +225,14 @@ export function ClassBody(...args: Array<any>): Object {
   return builder("ClassBody", ...args);
 }
 export { ClassBody as classBody };
-export function ClassDeclaration(...args: Array<any>): Object {
-  return builder("ClassDeclaration", ...args);
-}
-export { ClassDeclaration as classDeclaration };
 export function ClassExpression(...args: Array<any>): Object {
   return builder("ClassExpression", ...args);
 }
 export { ClassExpression as classExpression };
+export function ClassDeclaration(...args: Array<any>): Object {
+  return builder("ClassDeclaration", ...args);
+}
+export { ClassDeclaration as classDeclaration };
 export function ExportAllDeclaration(...args: Array<any>): Object {
   return builder("ExportAllDeclaration", ...args);
 }
diff --git a/packages/babel-types/src/constants/index.js b/packages/babel-types/src/constants/index.js
index b777caa0f11e..4dfe13d65adb 100644
--- a/packages/babel-types/src/constants/index.js
+++ b/packages/babel-types/src/constants/index.js
@@ -41,6 +41,12 @@ export const BINARY_OPERATORS = [
   ...BOOLEAN_BINARY_OPERATORS,
 ];
 
+export const ASSIGNMENT_OPERATORS = [
+  "=",
+  "+=",
+  ...NUMBER_BINARY_OPERATORS.map(op => op + "="),
+];
+
 export const BOOLEAN_UNARY_OPERATORS = ["delete", "!"];
 export const NUMBER_UNARY_OPERATORS = ["+", "-", "~"];
 export const STRING_UNARY_OPERATORS = ["typeof"];
diff --git a/packages/babel-types/src/definitions/core.js b/packages/babel-types/src/definitions/core.js
index 973c3bfa67c8..9461e36d3cc2 100644
--- a/packages/babel-types/src/definitions/core.js
+++ b/packages/babel-types/src/definitions/core.js
@@ -1,9 +1,13 @@
 // @flow
-import isValidIdentifier from "../validators/isValidIdentifier";
+
+import esutils from "esutils";
+
+import is from "../validators/is";
 
 import {
   BINARY_OPERATORS,
   LOGICAL_OPERATORS,
+  ASSIGNMENT_OPERATORS,
   UNARY_OPERATORS,
   UPDATE_OPERATORS,
 } from "../constants";
@@ -26,7 +30,7 @@ defineType("ArrayExpression", {
           assertNodeOrValueType("null", "Expression", "SpreadElement"),
         ),
       ),
-      default: [],
+      default: !process.env.BABEL_TYPES_8_BREAKING ? [] : undefined,
     },
   },
   visitor: ["elements"],
@@ -36,10 +40,29 @@ defineType("ArrayExpression", {
 defineType("AssignmentExpression", {
   fields: {
     operator: {
-      validate: assertValueType("string"),
+      validate: (function() {
+        if (!process.env.BABEL_TYPES_8_BREAKING) {
+          return assertValueType("string");
+        }
+
+        const identifier = assertOneOf(...ASSIGNMENT_OPERATORS);
+        const pattern = assertOneOf("=");
+
+        return function(node, key, val) {
+          const validator = is("Pattern", node.left) ? pattern : identifier;
+          validator(node, key, val);
+        };
+      })(),
     },
     left: {
-      validate: assertNodeType("LVal"),
+      validate: !process.env.BABEL_TYPES_8_BREAKING
+        ? assertNodeType("LVal")
+        : assertNodeType(
+            "Identifier",
+            "MemberExpression",
+            "ArrayPattern",
+            "ObjectPattern",
+          ),
     },
     right: {
       validate: assertNodeType("Expression"),
@@ -147,10 +170,14 @@ defineType("CallExpression", {
         ),
       ),
     },
-    optional: {
-      validate: assertOneOf(true, false),
-      optional: true,
-    },
+    ...(!process.env.BABEL_TYPES_8_BREAKING
+      ? {
+          optional: {
+            validate: assertOneOf(true, false),
+            optional: true,
+          },
+        }
+      : {}),
     typeArguments: {
       validate: assertNodeType("TypeParameterInstantiation"),
       optional: true,
@@ -166,7 +193,7 @@ defineType("CatchClause", {
   visitor: ["param", "body"],
   fields: {
     param: {
-      validate: assertNodeType("Identifier"),
+      validate: assertNodeType("Identifier", "ArrayPattern", "ObjectPattern"),
       optional: true,
     },
     body: {
@@ -256,7 +283,15 @@ defineType("ForInStatement", {
   ],
   fields: {
     left: {
-      validate: assertNodeType("VariableDeclaration", "LVal"),
+      validate: !process.env.BABEL_TYPES_8_BREAKING
+        ? assertNodeType("VariableDeclaration", "LVal")
+        : assertNodeType(
+            "VariableDeclaration",
+            "Identifier",
+            "MemberExpression",
+            "ArrayPattern",
+            "ObjectPattern",
+          ),
     },
     right: {
       validate: assertNodeType("Expression"),
@@ -305,10 +340,8 @@ export const functionCommon = {
   },
   generator: {
     default: false,
-    validate: assertValueType("boolean"),
   },
   async: {
-    validate: assertValueType("boolean"),
     default: false,
   },
 };
@@ -359,6 +392,17 @@ defineType("FunctionDeclaration", {
     "Pureish",
     "Declaration",
   ],
+  validate: (function() {
+    if (!process.env.BABEL_TYPES_8_BREAKING) return () => {};
+
+    const identifier = assertNodeType("Identifier");
+
+    return function(parent, key, node) {
+      if (!is("ExportDefaultDeclaration", parent)) {
+        identifier(node, "id", node.id);
+      }
+    };
+  })(),
 });
 
 defineType("FunctionExpression", {
@@ -405,17 +449,55 @@ defineType("Identifier", {
   fields: {
     ...patternLikeCommon,
     name: {
-      validate: chain(function(node, key, val) {
-        if (!isValidIdentifier(val)) {
-          // throw new TypeError(`"${val}" is not a valid identifier name`);
+      validate: chain(assertValueType("string"), function(node, key, val) {
+        if (!process.env.BABEL_TYPES_8_BREAKING) return;
+
+        if (!esutils.keyword.isIdentifierNameES6(val)) {
+          throw new TypeError(`"${val}" is not a valid identifier name`);
         }
-      }, assertValueType("string")),
+      }),
     },
     optional: {
       validate: assertValueType("boolean"),
       optional: true,
     },
   },
+  validate(parent, key, node) {
+    if (!process.env.BABEL_TYPES_8_BREAKING) return;
+
+    const match = /\.(\w+)$/.exec(key);
+    if (!match) return;
+
+    const [, parentKey] = match;
+    const nonComp = { computed: false };
+
+    // We can't check if `parent.property === node`, because nodes are validated
+    // before replacing them in the AST.
+    if (parentKey === "property") {
+      if (is("MemberExpression", parent, nonComp)) return;
+      if (is("OptionalMemberExpression", parent, nonComp)) return;
+    } else if (parentKey === "key") {
+      if (is("Property", parent, nonComp)) return;
+      if (is("Method", parent, nonComp)) return;
+    } else if (parentKey === "exported") {
+      if (is("ExportSpecifier", parent)) return;
+    } else if (parentKey === "imported") {
+      if (is("ImportSpecifier", parent, { imported: node })) return;
+    } else if (parentKey === "meta") {
+      if (is("MetaProperty", parent, { meta: node })) return;
+    }
+
+    if (
+      // Ideally this should be strict if this node is a descendant of a block
+      // in strict mode. Also, we should disallow "await" in modules.
+      esutils.keyword.isReservedWordES6(node.name, /* strict */ false) &&
+      // Even if "this" is a keyword, we are using the Identifier
+      // node to represent it.
+      node.name !== "this"
+    ) {
+      throw new TypeError(`"${node.name}" is not a valid identifer`);
+    }
+  },
 });
 
 defineType("IfStatement", {
@@ -492,7 +574,14 @@ defineType("RegExpLiteral", {
       validate: assertValueType("string"),
     },
     flags: {
-      validate: assertValueType("string"),
+      validate: chain(assertValueType("string"), function(node, key, val) {
+        if (!process.env.BABEL_TYPES_8_BREAKING) return;
+
+        const invalid = /[^gimsuy]/.exec(val);
+        if (invalid) {
+          throw new TypeError(`"${invalid[0]}" is not a valid RegExp flag`);
+        }
+      }),
       default: "",
     },
   },
@@ -537,10 +626,14 @@ defineType("MemberExpression", {
     computed: {
       default: false,
     },
-    optional: {
-      validate: assertOneOf(true, false),
-      optional: true,
-    },
+    ...(!process.env.BABEL_TYPES_8_BREAKING
+      ? {
+          optional: {
+            validate: assertOneOf(true, false),
+            optional: true,
+          },
+        }
+      : {}),
   },
 });
 
@@ -597,19 +690,15 @@ defineType("ObjectExpression", {
 });
 
 defineType("ObjectMethod", {
-  builder: ["kind", "key", "params", "body", "computed"],
+  builder: ["kind", "key", "params", "body", "computed", "generator", "async"],
   fields: {
     ...functionCommon,
     ...functionTypeAnnotationCommon,
     kind: {
-      validate: chain(
-        assertValueType("string"),
-        assertOneOf("method", "get", "set"),
-      ),
-      default: "method",
+      validate: assertOneOf("method", "get", "set"),
+      ...(!process.env.BABEL_TYPES_8_BREAKING ? { default: "method" } : {}),
     },
     computed: {
-      validate: assertValueType("boolean"),
       default: false,
     },
     key: {
@@ -632,6 +721,7 @@ defineType("ObjectMethod", {
         assertValueType("array"),
         assertEach(assertNodeType("Decorator")),
       ),
+      optional: true,
     },
     body: {
       validate: assertNodeType("BlockStatement"),
@@ -657,10 +747,15 @@ defineType("ObjectMethod", {
 });
 
 defineType("ObjectProperty", {
-  builder: ["key", "value", "computed", "shorthand", "decorators"],
+  builder: [
+    "key",
+    "value",
+    "computed",
+    "shorthand",
+    ...(!process.env.BABEL_TYPES_8_BREAKING ? ["decorators"] : []),
+  ],
   fields: {
     computed: {
-      validate: assertValueType("boolean"),
       default: false,
     },
     key: {
@@ -684,7 +779,27 @@ defineType("ObjectProperty", {
       validate: assertNodeType("Expression", "PatternLike"),
     },
     shorthand: {
-      validate: assertValueType("boolean"),
+      validate: chain(
+        assertValueType("boolean"),
+        function(node, key, val) {
+          if (!process.env.BABEL_TYPES_8_BREAKING) return;
+
+          if (val && node.computed) {
+            throw new TypeError(
+              "Property shorthand of ObjectProperty cannot be true if computed is true",
+            );
+          }
+        },
+        function(node, key, val) {
+          if (!process.env.BABEL_TYPES_8_BREAKING) return;
+
+          if (val && !is("Identifier", node.key)) {
+            throw new TypeError(
+              "Property shorthand of ObjectProperty cannot be true if key is not an Identifier",
+            );
+          }
+        },
+      ),
       default: false,
     },
     decorators: {
@@ -697,6 +812,17 @@ defineType("ObjectProperty", {
   },
   visitor: ["key", "value", "decorators"],
   aliases: ["UserWhitespacable", "Property", "ObjectMember"],
+  validate: (function() {
+    const pattern = assertNodeType("Identifier", "Pattern");
+    const expression = assertNodeType("Expression");
+
+    return function(parent, key, node) {
+      if (!process.env.BABEL_TYPES_8_BREAKING) return;
+
+      const validator = is("ObjectPattern", parent) ? pattern : expression;
+      validator(node, "value", node.value);
+    };
+  })(),
 });
 
 defineType("RestElement", {
@@ -707,9 +833,22 @@ defineType("RestElement", {
   fields: {
     ...patternLikeCommon,
     argument: {
-      validate: assertNodeType("LVal"),
+      validate: !process.env.BABEL_TYPES_8_BREAKING
+        ? assertNodeType("LVal")
+        : assertNodeType("Identifier", "Pattern", "MemberExpression"),
     },
   },
+  validate(parent, key) {
+    if (!process.env.BABEL_TYPES_8_BREAKING) return;
+
+    const match = /(\w+)\[(\d+)\]/.exec(key);
+    if (!match) throw new Error("Internal Babel error: malformed key.");
+
+    const [, listKey, index] = match;
+    if (parent[listKey].length > index + 1) {
+      throw new TypeError(`RestElement must be last element of ${listKey}`);
+    }
+  },
 });
 
 defineType("ReturnStatement", {
@@ -792,13 +931,23 @@ defineType("ThrowStatement", {
   },
 });
 
-// todo: at least handler or finalizer should be set to be valid
 defineType("TryStatement", {
   visitor: ["block", "handler", "finalizer"],
   aliases: ["Statement"],
   fields: {
     block: {
-      validate: assertNodeType("BlockStatement"),
+      validate: chain(assertNodeType("BlockStatement"), function(node) {
+        if (!process.env.BABEL_TYPES_8_BREAKING) return;
+
+        // This validator isn't put at the top level because we can run it
+        // even if this node doesn't have a parent.
+
+        if (!node.handler && !node.finalizer) {
+          throw new TypeError(
+            "TryStatement expects either a handler or finalizer, or both",
+          );
+        }
+      }),
     },
     handler: {
       optional: true,
@@ -835,7 +984,9 @@ defineType("UpdateExpression", {
       default: false,
     },
     argument: {
-      validate: assertNodeType("Expression"),
+      validate: !process.env.BABEL_TYPES_8_BREAKING
+        ? assertNodeType("Expression")
+        : assertNodeType("Identifier", "MemberExpression"),
     },
     operator: {
       validate: assertOneOf(...UPDATE_OPERATORS),
@@ -855,10 +1006,7 @@ defineType("VariableDeclaration", {
       optional: true,
     },
     kind: {
-      validate: chain(
-        assertValueType("string"),
-        assertOneOf("var", "let", "const"),
-      ),
+      validate: assertOneOf("var", "let", "const"),
     },
     declarations: {
       validate: chain(
@@ -867,13 +1015,39 @@ defineType("VariableDeclaration", {
       ),
     },
   },
+  validate(parent, key, node) {
+    if (!process.env.BABEL_TYPES_8_BREAKING) return;
+
+    if (!is("ForXStatement", parent, { left: node })) return;
+    if (node.declarations.length !== 1) {
+      throw new TypeError(
+        `Exactly one VariableDeclarator is required in the VariableDeclaration of a ${parent.type}`,
+      );
+    }
+  },
 });
 
 defineType("VariableDeclarator", {
   visitor: ["id", "init"],
   fields: {
     id: {
-      validate: assertNodeType("LVal"),
+      validate: (function() {
+        if (!process.env.BABEL_TYPES_8_BREAKING) {
+          return assertNodeType("LVal");
+        }
+
+        const normal = assertNodeType(
+          "Identifier",
+          "ArrayPattern",
+          "ObjectPattern",
+        );
+        const without = assertNodeType("Identifier");
+
+        return function(node, key, val) {
+          const validator = node.init ? normal : without;
+          validator(node, key, val);
+        };
+      })(),
     },
     definite: {
       optional: true,
@@ -894,7 +1068,7 @@ defineType("WhileStatement", {
       validate: assertNodeType("Expression"),
     },
     body: {
-      validate: assertNodeType("BlockStatement", "Statement"),
+      validate: assertNodeType("Statement"),
     },
   },
 });
@@ -907,7 +1081,7 @@ defineType("WithStatement", {
       validate: assertNodeType("Expression"),
     },
     body: {
-      validate: assertNodeType("BlockStatement", "Statement"),
+      validate: assertNodeType("Statement"),
     },
   },
 });
diff --git a/packages/babel-types/src/definitions/es2015.js b/packages/babel-types/src/definitions/es2015.js
index 53f3c1f3df7d..853af1b12e65 100644
--- a/packages/babel-types/src/definitions/es2015.js
+++ b/packages/babel-types/src/definitions/es2015.js
@@ -3,6 +3,7 @@ import defineType, {
   assertShape,
   assertNodeType,
   assertValueType,
+  assertNodeOrValueType,
   chain,
   assertEach,
   assertOneOf,
@@ -13,6 +14,7 @@ import {
   functionTypeAnnotationCommon,
   patternLikeCommon,
 } from "./core";
+import is from "../validators/is";
 
 defineType("AssignmentPattern", {
   visitor: ["left", "right", "decorators" /* for legacy param decorators */],
@@ -31,11 +33,13 @@ defineType("AssignmentPattern", {
     right: {
       validate: assertNodeType("Expression"),
     },
+    // For TypeScript
     decorators: {
       validate: chain(
         assertValueType("array"),
         assertEach(assertNodeType("Decorator")),
       ),
+      optional: true,
     },
   },
 });
@@ -49,14 +53,16 @@ defineType("ArrayPattern", {
     elements: {
       validate: chain(
         assertValueType("array"),
-        assertEach(assertNodeType("PatternLike")),
+        assertEach(assertNodeOrValueType("null", "PatternLike")),
       ),
     },
+    // For TypeScript
     decorators: {
       validate: chain(
         assertValueType("array"),
         assertEach(assertNodeType("Decorator")),
       ),
+      optional: true,
     },
   },
 });
@@ -106,41 +112,7 @@ defineType("ClassBody", {
   },
 });
 
-const classCommon = {
-  typeParameters: {
-    validate: assertNodeType(
-      "TypeParameterDeclaration",
-      "TSTypeParameterDeclaration",
-      "Noop",
-    ),
-    optional: true,
-  },
-  body: {
-    validate: assertNodeType("ClassBody"),
-  },
-  superClass: {
-    optional: true,
-    validate: assertNodeType("Expression"),
-  },
-  superTypeParameters: {
-    validate: assertNodeType(
-      "TypeParameterInstantiation",
-      "TSTypeParameterInstantiation",
-    ),
-    optional: true,
-  },
-  implements: {
-    validate: chain(
-      assertValueType("array"),
-      assertEach(
-        assertNodeType("TSExpressionWithTypeArguments", "ClassImplements"),
-      ),
-    ),
-    optional: true,
-  },
-};
-
-defineType("ClassDeclaration", {
+defineType("ClassExpression", {
   builder: ["id", "superClass", "body", "decorators"],
   visitor: [
     "id",
@@ -152,40 +124,22 @@ defineType("ClassDeclaration", {
     "implements",
     "decorators",
   ],
-  aliases: ["Scopable", "Class", "Statement", "Declaration", "Pureish"],
+  aliases: ["Scopable", "Class", "Expression", "Pureish"],
   fields: {
-    ...classCommon,
-    declare: {
-      validate: assertValueType("boolean"),
-      optional: true,
-    },
-    abstract: {
-      validate: assertValueType("boolean"),
-      optional: true,
-    },
     id: {
       validate: assertNodeType("Identifier"),
-      optional: true, // Missing if this is the child of an ExportDefaultDeclaration.
+      // In declarations, this is missing if this is the
+      // child of an ExportDefaultDeclaration.
+      optional: true,
     },
-    decorators: {
-      validate: chain(
-        assertValueType("array"),
-        assertEach(assertNodeType("Decorator")),
+    typeParameters: {
+      validate: assertNodeType(
+        "TypeParameterDeclaration",
+        "TSTypeParameterDeclaration",
+        "Noop",
       ),
       optional: true,
     },
-  },
-});
-
-defineType("ClassExpression", {
-  inherits: "ClassDeclaration",
-  aliases: ["Scopable", "Class", "Expression", "Pureish"],
-  fields: {
-    ...classCommon,
-    id: {
-      optional: true,
-      validate: assertNodeType("Identifier"),
-    },
     body: {
       validate: assertNodeType("ClassBody"),
     },
@@ -193,6 +147,22 @@ defineType("ClassExpression", {
       optional: true,
       validate: assertNodeType("Expression"),
     },
+    superTypeParameters: {
+      validate: assertNodeType(
+        "TypeParameterInstantiation",
+        "TSTypeParameterInstantiation",
+      ),
+      optional: true,
+    },
+    implements: {
+      validate: chain(
+        assertValueType("array"),
+        assertEach(
+          assertNodeType("TSExpressionWithTypeArguments", "ClassImplements"),
+        ),
+      ),
+      optional: true,
+    },
     decorators: {
       validate: chain(
         assertValueType("array"),
@@ -203,6 +173,32 @@ defineType("ClassExpression", {
   },
 });
 
+defineType("ClassDeclaration", {
+  inherits: "ClassExpression",
+  aliases: ["Scopable", "Class", "Statement", "Declaration", "Pureish"],
+  fields: {
+    declare: {
+      validate: assertValueType("boolean"),
+      optional: true,
+    },
+    abstract: {
+      validate: assertValueType("boolean"),
+      optional: true,
+    },
+  },
+  validate: (function() {
+    const identifier = assertNodeType("Identifier");
+
+    return function(parent, key, node) {
+      if (!process.env.BABEL_TYPES_8_BREAKING) return;
+
+      if (!is("ExportDefaultDeclaration", parent)) {
+        identifier(node, "id", node.id);
+      }
+    };
+  })(),
+});
+
 defineType("ExportAllDeclaration", {
   visitor: ["source"],
   aliases: [
@@ -248,18 +244,53 @@ defineType("ExportNamedDeclaration", {
   ],
   fields: {
     declaration: {
-      validate: assertNodeType("Declaration"),
       optional: true,
+      validate: chain(
+        assertNodeType("Declaration"),
+        function(node, key, val) {
+          if (!process.env.BABEL_TYPES_8_BREAKING) return;
+
+          // This validator isn't put at the top level because we can run it
+          // even if this node doesn't have a parent.
+
+          if (val && node.specifiers.length) {
+            throw new TypeError(
+              "Only declaration or specifiers is allowed on ExportNamedDeclaration",
+            );
+          }
+        },
+        function(node, key, val) {
+          if (!process.env.BABEL_TYPES_8_BREAKING) return;
+
+          // This validator isn't put at the top level because we can run it
+          // even if this node doesn't have a parent.
+
+          if (val && node.source) {
+            throw new TypeError("Cannot export a declaration from a source");
+          }
+        },
+      ),
     },
     specifiers: {
+      default: [],
       validate: chain(
         assertValueType("array"),
         assertEach(
-          assertNodeType(
-            "ExportSpecifier",
-            "ExportDefaultSpecifier",
-            "ExportNamespaceSpecifier",
-          ),
+          (function() {
+            const sourced = assertNodeType(
+              "ExportSpecifier",
+              "ExportDefaultSpecifier",
+              "ExportNamespaceSpecifier",
+            );
+            const sourceless = assertNodeType("ExportSpecifier");
+
+            if (!process.env.BABEL_TYPES_8_BREAKING) return sourced;
+
+            return function(node, key, val) {
+              const validator = node.source ? sourced : sourceless;
+              validator(node, key, val);
+            };
+          })(),
         ),
       ),
     },
@@ -286,6 +317,7 @@ defineType("ExportSpecifier", {
 
 defineType("ForOfStatement", {
   visitor: ["left", "right", "body"],
+  builder: ["left", "right", "body", "await"],
   aliases: [
     "Scopable",
     "Statement",
@@ -296,7 +328,27 @@ defineType("ForOfStatement", {
   ],
   fields: {
     left: {
-      validate: assertNodeType("VariableDeclaration", "LVal"),
+      validate: (function() {
+        if (!process.env.BABEL_TYPES_8_BREAKING) {
+          return assertNodeType("VariableDeclaration", "LVal");
+        }
+
+        const declaration = assertNodeType("VariableDeclaration");
+        const lval = assertNodeType(
+          "Identifier",
+          "MemberExpression",
+          "ArrayPattern",
+          "ObjectPattern",
+        );
+
+        return function(node, key, val) {
+          if (is("VariableDeclaration", val)) {
+            declaration(node, key, val);
+          } else {
+            lval(node, key, val);
+          }
+        };
+      })(),
     },
     right: {
       validate: assertNodeType("Expression"),
@@ -306,7 +358,6 @@ defineType("ForOfStatement", {
     },
     await: {
       default: false,
-      validate: assertValueType("boolean"),
     },
   },
 });
@@ -380,9 +431,26 @@ defineType("MetaProperty", {
   visitor: ["meta", "property"],
   aliases: ["Expression"],
   fields: {
-    // todo: limit to new.target
     meta: {
-      validate: assertNodeType("Identifier"),
+      validate: chain(assertNodeType("Identifier"), function(node, key, val) {
+        if (!process.env.BABEL_TYPES_8_BREAKING) return;
+
+        let property;
+        switch (val.name) {
+          case "function":
+            property = "sent";
+            break;
+          case "new":
+            property = "target";
+            break;
+          case "import":
+            property = "meta";
+            break;
+        }
+        if (!is("Identifier", node.property, { name: property })) {
+          throw new TypeError("Unrecognised MetaProperty");
+        }
+      }),
     },
     property: {
       validate: assertNodeType("Identifier"),
@@ -396,19 +464,14 @@ export const classMethodOrPropertyCommon = {
     optional: true,
   },
   accessibility: {
-    validate: chain(
-      assertValueType("string"),
-      assertOneOf("public", "private", "protected"),
-    ),
+    validate: assertOneOf("public", "private", "protected"),
     optional: true,
   },
   static: {
     default: false,
-    validate: assertValueType("boolean"),
   },
   computed: {
     default: false,
-    validate: assertValueType("boolean"),
   },
   optional: {
     validate: assertValueType("boolean"),
@@ -443,10 +506,7 @@ export const classMethodOrDeclareMethodCommon = {
   ...functionCommon,
   ...classMethodOrPropertyCommon,
   kind: {
-    validate: chain(
-      assertValueType("string"),
-      assertOneOf("get", "set", "method", "constructor"),
-    ),
+    validate: assertOneOf("get", "set", "method", "constructor"),
     default: "method",
   },
   access: {
@@ -467,7 +527,16 @@ export const classMethodOrDeclareMethodCommon = {
 
 defineType("ClassMethod", {
   aliases: ["Function", "Scopable", "BlockParent", "FunctionParent", "Method"],
-  builder: ["kind", "key", "params", "body", "computed", "static"],
+  builder: [
+    "kind",
+    "key",
+    "params",
+    "body",
+    "computed",
+    "static",
+    "generator",
+    "async",
+  ],
   visitor: [
     "key",
     "params",
@@ -554,7 +623,6 @@ defineType("TemplateElement", {
       }),
     },
     tail: {
-      validate: assertValueType("boolean"),
       default: false,
     },
   },
@@ -595,7 +663,15 @@ defineType("YieldExpression", {
   aliases: ["Expression", "Terminatorless"],
   fields: {
     delegate: {
-      validate: assertValueType("boolean"),
+      validate: chain(assertValueType("boolean"), function(node, key, val) {
+        if (!process.env.BABEL_TYPES_8_BREAKING) return;
+
+        if (val && !node.argument) {
+          throw new TypeError(
+            "Property delegate of YieldExpression cannot be true if there is no argument",
+          );
+        }
+      }),
       default: false,
     },
     argument: {
diff --git a/packages/babel-types/src/definitions/experimental.js b/packages/babel-types/src/definitions/experimental.js
index f4e110e1002f..a77350105a5b 100644
--- a/packages/babel-types/src/definitions/experimental.js
+++ b/packages/babel-types/src/definitions/experimental.js
@@ -26,9 +26,16 @@ defineType("AwaitExpression", {
 defineType("BindExpression", {
   visitor: ["object", "callee"],
   aliases: ["Expression"],
-  fields: {
-    // todo
-  },
+  fields: !process.env.BABEL_TYPES_8_BREAKING
+    ? {}
+    : {
+        object: {
+          validate: assertNodeType("Expression"),
+        },
+        callee: {
+          validate: assertNodeType("Expression"),
+        },
+      },
 });
 
 defineType("ClassProperty", {
diff --git a/packages/babel-types/src/definitions/flow.js b/packages/babel-types/src/definitions/flow.js
index 68560582605f..e04d16886d07 100644
--- a/packages/babel-types/src/definitions/flow.js
+++ b/packages/babel-types/src/definitions/flow.js
@@ -468,7 +468,7 @@ defineType("VoidTypeAnnotation", {
 
 // Enums
 defineType("EnumDeclaration", {
-  alises: ["Declaration"],
+  aliases: ["Declaration"],
   visitor: ["id", "body"],
   fields: {
     id: validateType("Identifier"),
diff --git a/packages/babel-types/src/definitions/index.js b/packages/babel-types/src/definitions/index.js
index d6e1763ebdf0..7341c67369a9 100644
--- a/packages/babel-types/src/definitions/index.js
+++ b/packages/babel-types/src/definitions/index.js
@@ -14,6 +14,7 @@ import {
   NODE_FIELDS,
   BUILDER_KEYS,
   DEPRECATED_KEYS,
+  NODE_PARENT_VALIDATIONS,
 } from "./utils";
 import {
   PLACEHOLDERS,
@@ -43,6 +44,7 @@ export {
   NODE_FIELDS,
   BUILDER_KEYS,
   DEPRECATED_KEYS,
+  NODE_PARENT_VALIDATIONS,
   PLACEHOLDERS,
   PLACEHOLDERS_ALIAS,
   PLACEHOLDERS_FLIPPED_ALIAS,
diff --git a/packages/babel-types/src/definitions/jsx.js b/packages/babel-types/src/definitions/jsx.js
index a088f03c1929..9b74dd52f488 100644
--- a/packages/babel-types/src/definitions/jsx.js
+++ b/packages/babel-types/src/definitions/jsx.js
@@ -142,7 +142,6 @@ defineType("JSXOpeningElement", {
     },
     selfClosing: {
       default: false,
-      validate: assertValueType("boolean"),
     },
     attributes: {
       validate: chain(
diff --git a/packages/babel-types/src/definitions/utils.js b/packages/babel-types/src/definitions/utils.js
index 3f7225bd50b2..cc3a347cca4a 100644
--- a/packages/babel-types/src/definitions/utils.js
+++ b/packages/babel-types/src/definitions/utils.js
@@ -1,6 +1,6 @@
 // @flow
 import is from "../validators/is";
-import { validateField } from "../validators/validate";
+import { validateField, validateChild } from "../validators/validate";
 
 export const VISITOR_KEYS: { [string]: Array<string> } = {};
 export const ALIAS_KEYS: { [string]: Array<string> } = {};
@@ -8,14 +8,13 @@ export const FLIPPED_ALIAS_KEYS: { [string]: Array<string> } = {};
 export const NODE_FIELDS: { [string]: {} } = {};
 export const BUILDER_KEYS: { [string]: Array<string> } = {};
 export const DEPRECATED_KEYS: { [string]: string } = {};
+export const NODE_PARENT_VALIDATIONS = {};
 
 function getType(val) {
   if (Array.isArray(val)) {
     return "array";
   } else if (val === null) {
     return "null";
-  } else if (val === undefined) {
-    return "undefined";
   } else {
     return typeof val;
   }
@@ -71,7 +70,10 @@ export function assertEach(callback: Validator): Validator {
     if (!Array.isArray(val)) return;
 
     for (let i = 0; i < val.length; i++) {
-      callback(node, `${key}[${i}]`, val[i]);
+      const subkey = `${key}[${i}]`;
+      const v = val[i];
+      callback(node, subkey, v);
+      if (process.env.BABEL_TYPES_8_BREAKING) validateChild(node, subkey, v);
     }
   }
   validator.each = callback;
@@ -96,23 +98,20 @@ export function assertOneOf(...values: Array<any>): Validator {
 
 export function assertNodeType(...types: Array<string>): Validator {
   function validate(node, key, val) {
-    let valid = false;
-
     for (const type of types) {
       if (is(type, val)) {
-        valid = true;
-        break;
+        validateChild(node, key, val);
+        return;
       }
     }
 
-    if (!valid) {
-      throw new TypeError(
-        `Property ${key} of ${
-          node.type
-        } expected node to be of a type ${JSON.stringify(types)} ` +
-          `but instead got ${JSON.stringify(val && val.type)}`,
-      );
-    }
+    throw new TypeError(
+      `Property ${key} of ${
+        node.type
+      } expected node to be of a type ${JSON.stringify(
+        types,
+      )} but instead got ${JSON.stringify(val && val.type)}`,
+    );
   }
 
   validate.oneOfNodeTypes = types;
@@ -122,23 +121,20 @@ export function assertNodeType(...types: Array<string>): Validator {
 
 export function assertNodeOrValueType(...types: Array<string>): Validator {
   function validate(node, key, val) {
-    let valid = false;
-
     for (const type of types) {
       if (getType(val) === type || is(type, val)) {
-        valid = true;
-        break;
+        validateChild(node, key, val);
+        return;
       }
     }
 
-    if (!valid) {
-      throw new TypeError(
-        `Property ${key} of ${
-          node.type
-        } expected node to be of a type ${JSON.stringify(types)} ` +
-          `but instead got ${JSON.stringify(val && val.type)}`,
-      );
-    }
+    throw new TypeError(
+      `Property ${key} of ${
+        node.type
+      } expected node to be of a type ${JSON.stringify(
+        types,
+      )} but instead got ${JSON.stringify(val && val.type)}`,
+    );
   }
 
   validate.oneOfNodeOrValueTypes = types;
@@ -200,6 +196,17 @@ export function chain(...fns: Array<Validator>): Validator {
   return validate;
 }
 
+const validTypeOpts = [
+  "aliases",
+  "builder",
+  "deprecatedAlias",
+  "fields",
+  "inherits",
+  "visitor",
+  "validate",
+];
+const validFieldKeys = ["default", "optional", "validate"];
+
 export default function defineType(
   type: string,
   opts: {
@@ -211,16 +218,38 @@ export default function defineType(
     builder?: Array<string>,
     inherits?: string,
     deprecatedAlias?: string,
+    validate?: Validator,
   } = {},
 ) {
   const inherits = (opts.inherits && store[opts.inherits]) || {};
 
-  const fields: Object = opts.fields || inherits.fields || {};
+  let fields = opts.fields;
+  if (!fields) {
+    fields = {};
+    if (inherits.fields) {
+      const keys = Object.getOwnPropertyNames(inherits.fields);
+      for (const key of (keys: Array<string>)) {
+        const field = inherits.fields[key];
+        fields[key] = {
+          default: field.default,
+          optional: field.optional,
+          validate: field.validate,
+        };
+      }
+    }
+  }
+
   const visitor: Array<string> = opts.visitor || inherits.visitor || [];
   const aliases: Array<string> = opts.aliases || inherits.aliases || [];
   const builder: Array<string> =
     opts.builder || inherits.builder || opts.visitor || [];
 
+  for (const k of (Object.keys(opts): Array<string>)) {
+    if (validTypeOpts.indexOf(k) === -1) {
+      throw new Error(`Unknown type option "${k}" on ${type}`);
+    }
+  }
+
   if (opts.deprecatedAlias) {
     DEPRECATED_KEYS[opts.deprecatedAlias] = type;
   }
@@ -233,14 +262,20 @@ export default function defineType(
   for (const key of Object.keys(fields)) {
     const field = fields[key];
 
-    if (builder.indexOf(key) === -1) {
+    if (field.default !== undefined && builder.indexOf(key) === -1) {
       field.optional = true;
     }
     if (field.default === undefined) {
       field.default = null;
-    } else if (!field.validate) {
+    } else if (!field.validate && field.default != null) {
       field.validate = assertValueType(getType(field.default));
     }
+
+    for (const k of (Object.keys(field): Array<string>)) {
+      if (validFieldKeys.indexOf(k) === -1) {
+        throw new Error(`Unknown field key "${k}" on ${type}.${key}`);
+      }
+    }
   }
 
   VISITOR_KEYS[type] = opts.visitor = visitor;
@@ -252,6 +287,10 @@ export default function defineType(
     FLIPPED_ALIAS_KEYS[alias].push(type);
   });
 
+  if (opts.validate) {
+    NODE_PARENT_VALIDATIONS[type] = opts.validate;
+  }
+
   store[type] = opts;
 }
 
diff --git a/packages/babel-types/src/validators/generated/index.js b/packages/babel-types/src/validators/generated/index.js
index 13ac0be73ac4..7c78934dfe8c 100644
--- a/packages/babel-types/src/validators/generated/index.js
+++ b/packages/babel-types/src/validators/generated/index.js
@@ -781,11 +781,11 @@ export function isClassBody(node: ?Object, opts?: Object): boolean {
 
   return false;
 }
-export function isClassDeclaration(node: ?Object, opts?: Object): boolean {
+export function isClassExpression(node: ?Object, opts?: Object): boolean {
   if (!node) return false;
 
   const nodeType = node.type;
-  if (nodeType === "ClassDeclaration") {
+  if (nodeType === "ClassExpression") {
     if (typeof opts === "undefined") {
       return true;
     } else {
@@ -795,11 +795,11 @@ export function isClassDeclaration(node: ?Object, opts?: Object): boolean {
 
   return false;
 }
-export function isClassExpression(node: ?Object, opts?: Object): boolean {
+export function isClassDeclaration(node: ?Object, opts?: Object): boolean {
   if (!node) return false;
 
   const nodeType = node.type;
-  if (nodeType === "ClassExpression") {
+  if (nodeType === "ClassDeclaration") {
     if (typeof opts === "undefined") {
       return true;
     } else {
@@ -3499,8 +3499,8 @@ export function isScopable(node: ?Object, opts?: Object): boolean {
     "SwitchStatement" === nodeType ||
     "WhileStatement" === nodeType ||
     "ArrowFunctionExpression" === nodeType ||
-    "ClassDeclaration" === nodeType ||
     "ClassExpression" === nodeType ||
+    "ClassDeclaration" === nodeType ||
     "ForOfStatement" === nodeType ||
     "ClassMethod" === nodeType ||
     "ClassPrivateMethod" === nodeType ||
@@ -3847,8 +3847,8 @@ export function isPureish(node: ?Object, opts?: Object): boolean {
     "NullLiteral" === nodeType ||
     "BooleanLiteral" === nodeType ||
     "ArrowFunctionExpression" === nodeType ||
-    "ClassDeclaration" === nodeType ||
     "ClassExpression" === nodeType ||
+    "ClassDeclaration" === nodeType ||
     "BigIntLiteral" === nodeType ||
     (nodeType === "Placeholder" && "StringLiteral" === node.expectedNode)
   ) {
@@ -3887,6 +3887,7 @@ export function isDeclaration(node: ?Object, opts?: Object): boolean {
     "InterfaceDeclaration" === nodeType ||
     "OpaqueType" === nodeType ||
     "TypeAlias" === nodeType ||
+    "EnumDeclaration" === nodeType ||
     "TSDeclareFunction" === nodeType ||
     "TSInterfaceDeclaration" === nodeType ||
     "TSTypeAliasDeclaration" === nodeType ||
@@ -4149,8 +4150,8 @@ export function isClass(node: ?Object, opts?: Object): boolean {
   const nodeType = node.type;
   if (
     nodeType === "Class" ||
-    "ClassDeclaration" === nodeType ||
-    "ClassExpression" === nodeType
+    "ClassExpression" === nodeType ||
+    "ClassDeclaration" === nodeType
   ) {
     if (typeof opts === "undefined") {
       return true;
diff --git a/packages/babel-types/src/validators/isValidIdentifier.js b/packages/babel-types/src/validators/isValidIdentifier.js
index 6dbbb7c32cd0..d8aaf8c36372 100644
--- a/packages/babel-types/src/validators/isValidIdentifier.js
+++ b/packages/babel-types/src/validators/isValidIdentifier.js
@@ -5,16 +5,20 @@ import esutils from "esutils";
  * Check if the input `name` is a valid identifier name
  * and isn't a reserved word.
  */
-export default function isValidIdentifier(name: string): boolean {
-  if (
-    typeof name !== "string" ||
-    esutils.keyword.isReservedWordES6(name, true)
-  ) {
-    return false;
-  } else if (name === "await") {
-    // invalid in module, valid in script; better be safe (see #4952)
-    return false;
-  } else {
-    return esutils.keyword.isIdentifierNameES6(name);
+export default function isValidIdentifier(
+  name: string,
+  reserved: boolean = true,
+): boolean {
+  if (typeof name !== "string") return false;
+
+  if (reserved) {
+    if (esutils.keyword.isReservedWordES6(name, true)) {
+      return false;
+    } else if (name === "await") {
+      // invalid in module, valid in script; better be safe (see #4952)
+      return false;
+    }
   }
+
+  return esutils.keyword.isIdentifierNameES6(name);
 }
diff --git a/packages/babel-types/src/validators/validate.js b/packages/babel-types/src/validators/validate.js
index 738683849927..0ec35b684a0f 100644
--- a/packages/babel-types/src/validators/validate.js
+++ b/packages/babel-types/src/validators/validate.js
@@ -1,5 +1,5 @@
 // @flow
-import { NODE_FIELDS } from "../definitions";
+import { NODE_FIELDS, NODE_PARENT_VALIDATIONS } from "../definitions";
 
 export default function validate(node?: Object, key: string, val: any): void {
   if (!node) return;
@@ -9,6 +9,7 @@ export default function validate(node?: Object, key: string, val: any): void {
 
   const field = fields[key];
   validateField(node, key, val, field);
+  validateChild(node, key, val);
 }
 
 export function validateField(
@@ -22,3 +23,10 @@ export function validateField(
 
   field.validate(node, key, val);
 }
+
+export function validateChild(node?: Object, key: string, val?: Object) {
+  if (val == null) return;
+  const validate = NODE_PARENT_VALIDATIONS[val.type];
+  if (!validate) return;
+  validate(node, key, val);
+}