Add flag for disallowing for-in initializers

test/mjsunit/harmony/for-in.js

Index: test/mjsunit/harmony/for-in.js
diff --git a/test/mjsunit/harmony/for-in.js b/test/mjsunit/harmony/for-in.js
new file mode 100644
index 0000000000000000000000000000000000000000..58e343b90355bbd8ad022463a18497d095e387c3
--- /dev/null
+++ b/test/mjsunit/harmony/for-in.js
@@ -0,0 +1,9 @@
+// Copyright 2015 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.
+
+// Flags: --harmony-for-in
+
+assertThrows("for (var x = 0 in {});", SyntaxError);
+assertThrows("for (const x = 0 in {});", SyntaxError);
+assertThrows("for (let x = 0 in {});", SyntaxError);