Use metadata annotation @patch for patch classes

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 {
+@patch class Null {
 
   factory Null._uninstantiable() {
     throw new UnsupportedError("class Null cannot be instantiated");
   }
 
   static const _HASH_CODE = 2011; // The year Dart was announced and a prime.
   int get hashCode => _HASH_CODE;
   int get _identityHashCode => _HASH_CODE;
 
   String toString() => 'null';
 }