[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..5f11e75ddc01341362fe0db34fe11e372edd003c 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."],
+  strict_for_in_loop_initializer: ["for-in loop variable declaration may not have an initializer in strict mode."],

[arv (Not doing code reviews), 2015/03/25 09:17:43]

This is misleading...

[caitp (gmail), 2015/03/25 11:22:42]

I know it's not technically allowed in sloppy mode, but I don't have telemetry
to know if it would break anyone. I'm more confident with strict-mode only (for
now) because of JSC is already doing that.

+  for_of_loop_initializer:       ["for-of loop variable declaration may not have an initializer."]
 };