Treat "await" as an invalid identifier (#4954)

It is valid (outside `async` functions) in the "script" parse goal, but always invalid in the "module" parse goal.

Fixes #4952.

packages/babel-plugin-transform-es2015-function-name/test/fixtures/function-name/await/actual.js

@@ -0,0 +1,3 @@
+export {};
+
+var obj = { await: function () {} };

packages/babel-plugin-transform-es2015-function-name/test/fixtures/function-name/await/expected.js

@@ -0,0 +1,3 @@
+export {};
+
+var obj = { await: function _await() {} };

packages/babel-types/src/validators.js

@@ -167,6 +167,9 @@ export function isReferenced(node: Object, parent: Object): boolean {
 export 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);
   }

packages/babel-types/test/validators.js

@@ -26,5 +26,9 @@ suite("validators", function () {
 
       assert(t.isNodesEquivalent(parse(program), parse(program2)) === false);
     });
+
+    it("rejects 'await' as an identifier", function () {
+      assert(t.isValidIdentifier("await") === false);
+    });
   });
 });