Fix another test

pkg/analysis_server/test/mocks.dart

Index: pkg/analysis_server/test/mocks.dart
diff --git a/pkg/analysis_server/test/mocks.dart b/pkg/analysis_server/test/mocks.dart
index 1265d3ef3671d6e172aa23e722b460ebc3335def..53f8458ae2026de0c282adb805010915d886cd5b 100644
--- a/pkg/analysis_server/test/mocks.dart
+++ b/pkg/analysis_server/test/mocks.dart
@@ -124,7 +124,7 @@ class NoResponseException implements Exception {
 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 = [];