minor fix to get dartcombat tests passing

frog/lib/isolate.js

 // 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.
 
 var isolate$current = null;
 var isolate$rootIsolate = null;  // Will only be set in the main worker.
 var isolate$inits = [];
 var isolate$globalThis = this;
 
 var isolate$inWorker =
@@ skipping 18 matching lines @@
 //   isolate$useWorkerSerializationProtocol = true;
 
 
 // ------- SendPort -------
 
 function isolate$receiveMessage(port, isolate,
                                 serializedMessage, serializedReplyTo) {
   isolate$IsolateEvent.enqueue(isolate, function() {
     var message = isolate$deserializeMessage(serializedMessage);
     var replyTo = isolate$deserializeMessage(serializedReplyTo);
-    port._callback(message, replyTo);
+    if (port._callback) port._callback(message, replyTo);
   });
 }
 
 // -------- Registry ---------
 function isolate$Registry() {
   this.map = {};
   this.count = 0;
 }
 
 isolate$Registry.prototype.register = function(id, val) {
@@ skipping 433 matching lines @@
 function isolate$deserializeMessage(message_) {
   if (isolate$useWorkers || isolate$useWorkerSerializationProtocol) {
     return _IsolateJsUtil._deserializeMessage(message_);
   } else {
     // Nothing more to do.
     return message_;
   }
 }
 
 function _IsolateJsUtil() {}