[es6]: Lexical arguments for arrow functions

src/scopes.h

Index: src/scopes.h
diff --git a/src/scopes.h b/src/scopes.h
index cce8c3c9eaa057558ac38bc953fe9b328719f40f..7b372d9643fe4c1c07892139c6ecf1937ed1257e 100644
--- a/src/scopes.h
+++ b/src/scopes.h
@@ -394,7 +394,10 @@ class Scope: public ZoneObject {
   }
 
   // The local variable 'arguments' if we need to allocate it; NULL otherwise.
-  Variable* arguments() const { return arguments_; }
+  Variable* arguments() const {
+    DCHECK(!is_arrow_scope() || arguments_ == nullptr);
+    return arguments_;
+  }
 
   // Declarations list.
   ZoneList<Declaration*>* declarations() { return &decls_; }