Make ReceivePort an abstract class.

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.
 
-patch class _ReceivePortFactory {
+patch class ReceivePort {
   /* patch */ factory ReceivePort() {
     return new _ReceivePortImpl();
   }
 }
 
 class _ReceivePortImpl implements ReceivePort {
   factory _ReceivePortImpl() native "ReceivePortImpl_factory";
 
   receive(void onMessage(var message, SendPort replyTo)) {
     _onMessage = onMessage;
@@ skipping 104 matching lines @@
   if (_portInternal == null) {
     _portInternal = _getPortInternal();
   }
   return _portInternal;
 }
 
 patch spawnFunction(void topLevelFunction()) native "isolate_spawnFunction";
 
 patch spawnUri(String uri) native "isolate_spawnUri";