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

test/message/for-of-let-loop-initializers.js

Index: test/message/for-of-let-loop-initializers.js
diff --git a/test/message/class-constructor-accessor.js b/test/message/for-of-let-loop-initializers.js
similarity index 71%
copy from test/message/class-constructor-accessor.js
copy to test/message/for-of-let-loop-initializers.js
index edc3c131697ed513a7c7a15c95e9aae2c68788ad..4ac0d549ce65c3750c9d0dd1d4c16d1f9d9214b9 100644
--- a/test/message/class-constructor-accessor.js
+++ b/test/message/for-of-let-loop-initializers.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 {
-  get constructor() {}
+function test() {
+  for (let x = void 0 of [1, 2, 3]) {
+    return x;
+  }
 }