Make __proto__ a foreign callback on Object.prototype.

test/mjsunit/harmony/proxies.js

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 2276 matching lines @@
 
 function TestConstructorWithProxyPrototype() {
   TestWithProxies(TestConstructorWithProxyPrototype2, {})
 }
 
 function TestConstructorWithProxyPrototype2(create, handler) {
   function C() {};
   C.prototype = create(handler);
 
   var o = new C;
-  assertSame(C.prototype, o.__proto__);
   assertSame(C.prototype, Object.getPrototypeOf(o));
 }
 
 TestConstructorWithProxyPrototype();