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

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

Index: test/message/for-in-let-loop-initializers-strict.out
diff --git a/test/message/for-in-let-loop-initializers-strict.out b/test/message/for-in-let-loop-initializers-strict.out
new file mode 100644
index 0000000000000000000000000000000000000000..6c8ca9dfe927ee73ffe3f84fdebde96a25f42724
--- /dev/null
+++ b/test/message/for-in-let-loop-initializers-strict.out
@@ -0,0 +1,7 @@
+# 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.
+*%(basename)s:8: SyntaxError: for-in loop variable declaration may not have an initializer.
+  for (let x = void 0 in [1, 2, 3]) {
+           ^^^^^^^^^^
+SyntaxError: for-in loop variable declaration may not have an initializer.