Use relative URIs for core library part-of statements.

sdk/lib/core/null.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.
 
-part of dart.core;
+part of "core.dart";
 
 /**
  * The reserved word [:null:] denotes an object that is the sole instance of 
  * this class.
  * 
  * It is a compile-time error for a class to attempt to extend or implement
  * Null.
  */
 class Null {
   factory Null._uninstantiable() {
     throw new UnsupportedError('class Null cannot be instantiated');
   }
 
   external int get hashCode;
 
   /** Returns the string `"null"`. */
   String toString() => "null";
 }