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

src/messages.js

Index: src/messages.js
diff --git a/src/messages.js b/src/messages.js
index 672fd4b8100fa26b499d983ab0d06a379d435793..d14cb600ae96a8ce675d16739c544378f0992c1b 100644
--- a/src/messages.js
+++ b/src/messages.js
@@ -197,7 +197,9 @@ var kMessages = {
   duplicate_proto:               ["Duplicate __proto__ fields are not allowed in object literals"],
   param_after_rest:              ["Rest parameter must be last formal parameter"],
   constructor_noncallable:       ["Class constructors cannot be invoked without 'new'"],
-  array_not_subclassable:        ["Subclassing Arrays is not currently supported."]
+  array_not_subclassable:        ["Subclassing Arrays is not currently supported."],
+  for_in_loop_initializer:       ["for-in loop variable declaration may not have an initializer."],
+  for_of_loop_initializer:       ["for-of loop variable declaration may not have an initializer."]
 };