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

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

Index: test/message/for-of-loop-initializers-sloppy.js
diff --git a/test/mjsunit/regress/regress-double-property.js b/test/message/for-of-loop-initializers-sloppy.js
similarity index 74%
copy from test/mjsunit/regress/regress-double-property.js
copy to test/message/for-of-loop-initializers-sloppy.js
index 2ddb45b4b66c79600c78abfaf5b64c1a9a13927a..685e2e6d2e52259b4bfe34329d88ebfb6508f2ce 100644
--- a/test/mjsunit/regress/regress-double-property.js
+++ b/test/message/for-of-loop-initializers-sloppy.js
@@ -1,9 +1,10 @@
 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
+//
 
-function f(a) {
-  return {0.1: a};
+function test() {
+  for (var x = void 0 of [1, 2, 3]) {
+    return x;
+  }
 }
-
-f();