[es6] emit error when for-in loop declarations are initialized in strict mode

test/message/for-in-let-loop-initializers-strict.js

Index: test/message/for-in-let-loop-initializers-strict.js
diff --git a/test/message/class-constructor-generator.js b/test/message/for-in-let-loop-initializers-strict.js
similarity index 71%
copy from test/message/class-constructor-generator.js
copy to test/message/for-in-let-loop-initializers-strict.js
index 5d370f865eaa84bd187aab7721acfc2a0a390eac..a58f2fd275a99c34e3c74c379b7ec268b83bc7e8 100644
--- a/test/message/class-constructor-generator.js
+++ b/test/message/for-in-let-loop-initializers-strict.js
@@ -2,9 +2,10 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 //
-// Flags: --harmony-classes
 'use strict';
 
-class C {
-  *constructor() {}
+function test() {
+  for (let x = void 0 in [1, 2, 3]) {
+    return x;
+  }
 }