Fix bogus mistake in get:_port().

runtime/lib/isolate.dart

 // Copyright (c) 2011, 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 _ReceivePortFactory {
   factory ReceivePort() {
     return new _ReceivePortImpl();
   }
 }
 
@@ skipping 117 matching lines @@
   // This method is being dispatched to from the public core library code.
   static SendPort _start(Isolate isolate, bool light)
       native "IsolateNatives_start";
 }
 
 _getPortInternal() native "isolate_getPortInternal";
 
 ReceivePort _portInternal;
 
 ReceivePort get _port() {
-  if (_portInternal) {
-    return _portInternal;
+  if (_portInternal === null) {
+    _portInternal = _getPortInternal();
   }
-  _portInternal = _getPortInternal();
   return _portInternal;
 }
 
 _spawnFunction(void topLevelFunction()) native "isolate_spawnFunction";
 
 // TODO(sigmund,turnidge): implement
 SendPort _spawnUri(String uri) {
   throw new NotImplementedException();
 }