Implement updated method overriding rules in the vm.

runtime/lib/isolate_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.
 
 class _CloseToken {
   /// This token is sent from [IsolateSink]s to [IsolateStream]s to ask them to
   /// close themselves.
   const _CloseToken();
 }
 
@@ skipping 169 matching lines @@
 ReceivePort _portInternal;
 
 patch class _Isolate {
   /* patch */ static ReceivePort get port {
     if (_portInternal == null) {
       _portInternal = _getPortInternal();
     }
     return _portInternal;
   }
 
-  /* patch */ static spawnFunction(void topLevelFunction(),
+  /* patch */ static SendPort spawnFunction(void topLevelFunction(),
       [bool unhandledExceptionCallback(IsolateUnhandledException e)])
       native "isolate_spawnFunction";
 
-  /* patch */ static spawnUri(String uri) native "isolate_spawnUri";
+  /* patch */ static SendPort spawnUri(String uri) native "isolate_spawnUri";
 }