Make Null a public class of dart:core.

runtime/lib/object_patch.dart

 // Copyright (c) 2012, 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.
 
 patch class Object {
 
   // Helpers used to implement hashCode. If a hashCode is used, we remember it
   // in a weak table in the VM. A new hashCode value is calculated using a
   // number generator.
   static final _hashCodeRnd = new Random();
 
   static _getHash(obj) native "Object_getHash";
   static _setHash(obj, hash) native "Object_setHash";
 
   /* patch */ int get hashCode {
-    if (this == null) {
-      return 2011;  // The year Dart was announced and a prime.
-    }
     var result = _getHash(this);
     if (result == 0) {
       // We want the hash to be a Smi value greater than 0.
       result = _hashCodeRnd.nextInt(0x40000000);
       while (result == 0) {
         result = _hashCodeRnd.nextInt(0x40000000);
       }
       _setHash(this, result);
     }
     return result;
@@ skipping 39 matching lines @@
     });
     return result;
   }
 
   int get _cid native "Object_cid";
 
   _leftShiftWithMask32(count, mask)  {
     return (this << count) & mask;
   }
 }