Correctly update bindings of decorated class declarations

This commit adds a small utility (Scope#updateOwnBinding) which
should be used whenever you replace a declaration with a new
delaration which defines the same binding (e.g. `class Foo {}` ->
`let Foo = class Foo {}`). Since our scope system isn't currently
synchrinized with the AST, doing this automatically would add too
much overhead to the `replaceWith*` functions.

That function probably needs to be used in many more places, but we
can add them when users find bugs related to non-updated bingins.

packages/babel-plugin-proposal-decorators/src/transformer-legacy.js

@@ -252,6 +252,9 @@ export default {
     const replacement = decoratedClassToExpression(path);
     if (replacement) {
       path.replaceWith(replacement);
+
+      const decl = path.get("declarations.0");
+      path.scope.updateOwnBinding(decl.node.id, path.node.kind, decl);
     }
   },
   ClassExpression(path, state) {

packages/babel-plugin-proposal-decorators/test/fixtures/legacy-regression/8559/input.mjs

@@ -0,0 +1,12 @@
+import {autobind} from 'core-decorators';
+
+export default function wrap() {
+	return function() {
+		class Foo {
+			@autobind
+			method() {}
+		}
+
+		return Foo;
+	};
+}

packages/babel-plugin-proposal-decorators/test/fixtures/legacy-regression/8559/options.json

@@ -0,0 +1,12 @@
+{
+  "plugins": [
+    ["proposal-decorators", { "legacy": true }]
+  ],
+  "presets": [
+    ["env", {
+      "targets": {
+        "node": 8
+      }
+    }]
+  ]
+}

packages/babel-plugin-proposal-decorators/test/fixtures/legacy-regression/8559/output.js

@@ -0,0 +1,22 @@
+"use strict";
+
+Object.defineProperty(exports, "__esModule", {
+  value: true
+});
+exports.default = wrap;
+
+var _coreDecorators = require("core-decorators");
+
+function _applyDecoratedDescriptor(target, property, decorators, descriptor, context) { var desc = {}; Object['ke' + 'ys'](descriptor).forEach(function (key) { desc[key] = descriptor[key]; }); desc.enumerable = !!desc.enumerable; desc.configurable = !!desc.configurable; if ('value' in desc || desc.initializer) { desc.writable = true; } desc = decorators.slice().reverse().reduce(function (desc, decorator) { return decorator(target, property, desc) || desc; }, desc); if (context && desc.initializer !== void 0) { desc.value = desc.initializer ? desc.initializer.call(context) : void 0; desc.initializer = undefined; } if (desc.initializer === void 0) { Object['define' + 'Property'](target, property, desc); desc = null; } return desc; }
+
+function wrap() {
+  return function () {
+    var _class;
+
+    let Foo = (_class = class Foo {
+      method() {}
+
+    }, (_applyDecoratedDescriptor(_class.prototype, "method", [_coreDecorators.autobind], Object.getOwnPropertyDescriptor(_class.prototype, "method"), _class.prototype)), _class);
+    return Foo;
+  };
+}

packages/babel-traverse/src/scope/index.js

@@ -944,6 +944,17 @@ export default class Scope {
     return this.parent && this.parent.hasBinding(name, noGlobals);
   }
 
+  updateOwnBinding(id: t.Identifier, kind: string, path: NodePath) {
+    if (!kind) throw new ReferenceError("no `kind`");
+
+    const binding = this.getOwnBinding(id.name);
+    if (!binding) throw new ReferenceError(`Unknown binding: ${id.name}`);
+
+    binding.identifier = id;
+    binding.kind = kind;
+    binding.path = path;
+  }
+
   /**
    * Move a binding of `name` to another `scope`.
    */