Add Object.hashCodeOf to get the default hashCode of an object.

runtime/lib/null_patch.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // Dart core library.
 
 patch class Null {
 
   factory Null._uninstantiable() {
     throw new UnsupportedError(
         "class Null cannot be instantiated");
   }
 
-  int get hashCode {
+  int get _identityHashCode {
     return 2011;  // The year Dart was announced and a prime.
   }
 
   String toString() {
     return 'null';
   }
 }