Fix another test

pkg/analysis_server/test/mocks.dart

 // Copyright (c) 2014, 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.
 
 library mocks;
 
 import 'dart:async';
 import 'dart:io';
 
 @MirrorsUsed(targets: 'mocks', override: '*')
@@ skipping 106 matching lines @@
     return "NoResponseException after request ${request.toJson()}";
   }
 }
 
 /**
  * A mock [ServerCommunicationChannel] for testing [AnalysisServer].
  */
 class MockServerChannel implements ServerCommunicationChannel {
   StreamController<Request> requestController = new StreamController<Request>();
   StreamController<Response> responseController = new StreamController<Response>();
-  StreamController<Notification> notificationController = new StreamController<Notification>();
+  StreamController<Notification> notificationController = new StreamController<Notification>(sync: true);
 
   List<Response> responsesReceived = [];
   List<Notification> notificationsReceived = [];
 
   MockServerChannel() {
   }
 
   @override
   void listen(void onRequest(Request request), {Function onError, void onDone()}) {
     requestController.stream.listen(onRequest, onError: onError, onDone: onDone);
@@ skipping 158 matching lines @@
     Response response = item;
     var id = response.id;
     RequestError error = response.error;
     mismatchDescription.add('has identifier "$id"');
     if (error == null) {
       mismatchDescription.add(' and has no error');
     }
     return mismatchDescription;
   }
 }