[proxies] Implement [[PreventExtensions]] and [[IsExtensible]].

src/js/v8natives.js

 // 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.
 
 (function(global, utils) {
 
 %CheckIsBootstrapping();
 
 // ----------------------------------------------------------------------------
 // Imports
@@ skipping 1203 matching lines @@
     if (IsDataDescriptor(desc) && desc.isWritable()) return false;
     if (desc.isConfigurable()) return false;
   }
   return true;
 }
 
 
 // ES5 section 15.2.3.13
 function ObjectIsExtensible(obj) {
   if (!IS_SPEC_OBJECT(obj)) return false;
-  if (%_IsJSProxy(obj)) {
-    return true;
-  }
   return %IsExtensible(obj);
 }
 
 
 // ECMA-262, Edition 6, section 19.1.2.1
 function ObjectAssign(target, sources) {
   // TODO(bmeurer): Move this to toplevel.
   "use strict";
   var to = TO_OBJECT(target);
   var argsLen = %_ArgumentsLength();
@@ skipping 574 matching lines @@
 
 %InstallToContext([
   "global_eval_fun", GlobalEval,
   "object_value_of", ObjectValueOf,
   "object_to_string", ObjectToString,
   "object_get_own_property_descriptor", ObjectGetOwnPropertyDescriptor,
   "to_complete_property_descriptor", ToCompletePropertyDescriptor,
 ]);
 
 })