Make __proto__ a foreign callback on Object.prototype.

test/mjsunit/harmony/object-observe.js

 // Copyright 2012 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 878 matching lines @@
 
 // __proto__
 reset();
 var obj = {};
 Object.observe(obj, observer.callback);
 var p = {foo: 'yes'};
 var q = {bar: 'no'};
 obj.__proto__ = p;
 obj.__proto__ = p;  // ignored
 obj.__proto__ = null;
-obj.__proto__ = q;
+obj.__proto__ = q;  // the __proto__ accessor is gone
 // TODO(adamk): Add tests for objects with hidden prototypes
 // once we support observing the global object.
 Object.deliverChangeRecords(observer.callback);
 observer.assertCallbackRecords([
   { object: obj, name: '__proto__', type: 'prototype',
     oldValue: Object.prototype },
   { object: obj, name: '__proto__', type: 'prototype', oldValue: p },
-  { object: obj, name: '__proto__', type: 'prototype', oldValue: null },
+  { object: obj, name: '__proto__', type: 'new' },
 ]);
 
 
 // Function.prototype
 reset();
 var fun = function(){};
 Object.observe(fun, observer.callback);
 var myproto = {foo: 'bar'};
 fun.prototype = myproto;
 fun.prototype = 7;
@@ skipping 127 matching lines @@
     for (var n1 = 0; n1 < 3; ++n1)
       for (var n2 = 0; n2 < 3; ++n2)
         for (var i in mutation)
           TestFastElementsLength(mutation[i], b1 != 0, b2 != 0, 20*n1, 20*n2);
 
 for (var b1 = 0; b1 < 2; ++b1)
   for (var b2 = 0; b2 < 2; ++b2)
     for (var n = 0; n < 3; ++n)
       for (var i in mutationByIncr)
         TestFastElementsLength(mutationByIncr[i], b1 != 0, b2 != 0, 7*n, 7*n+1);