[es6]: Lexical arguments for arrow functions

src/scopes.h

 // Copyright 2012 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.
 
 #ifndef V8_SCOPES_H_
 #define V8_SCOPES_H_
 
 #include "src/ast.h"
 #include "src/pending-compilation-error-handler.h"
 #include "src/zone.h"
@@ skipping 376 matching lines @@
     return rest_index_ >= 0;
   }
 
   bool is_simple_parameter_list() const {
     DCHECK(is_function_scope());
     if (rest_index_ >= 0) return false;
     return true;
   }
 
   // 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_; }
 
   // Inner scope list.
   ZoneList<Scope*>* inner_scopes() { return &inner_scopes_; }
 
   // The scope immediately surrounding this scope, or NULL.
   Scope* outer_scope() const { return outer_scope_; }
 
@@ skipping 319 matching lines @@
 
   AstValueFactory* ast_value_factory_;
   Zone* zone_;
 
   PendingCompilationErrorHandler pending_error_handler_;
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_SCOPES_H_