Fix http://dartbug.com/14763

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 ReceivePort {
   /* patch */ factory ReceivePort() = _ReceivePortImpl;
 
   /* patch */ factory ReceivePort.fromRawReceivePort(RawReceivePort rawPort) =
       _ReceivePortImpl.fromRawReceivePort;
 }
@@ skipping 156 matching lines @@
     // initial startup message has been received.
   }
 
   isolateStartHandler(message) {
     // We received the initial startup message. Ignore all further messages and
     // close the port which kept this isolate alive.
     Isolate._self.handler = ignoreHandler;
     keepAlivePort.close();
 
     SendPort replyTo = message[0];
-    // TODO(floitsch): don't send ok-message if we can't find the entry point.
-    replyTo.send("started");
+    if (replyTo != null) {
+      // TODO(floitsch): don't send ok-message if we can't find the entry point.
+      replyTo.send("started");
+    }
     if (isSpawnUri) {
       assert(message.length == 3);
       List<String> args = message[1];
       var isolateMessage = message[2];
       if (entryPoint is _MainFunctionArgsMessage) {
         entryPoint(args, isolateMessage);
       } else if (entryPoint is _MainFunctionArgs) {
         entryPoint(args);
       } else {
         entryPoint();
@@ skipping 50 matching lines @@
   }
 
   static final RawReceivePort _self = _mainPort;
   static RawReceivePort get _mainPort native "Isolate_mainPort";
 
   static SendPort _spawnFunction(Function topLevelFunction)
       native "Isolate_spawnFunction";
 
   static SendPort _spawnUri(String uri) native "Isolate_spawnUri";
 }