Server code clean-up

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 'package:analysis_server/src/channel.dart';
 
 /**
  * Instances of the class [MockChannel] implement a [CommunicationChannel] that
  * does nothing in response to every method invoked on it.
  */
 class MockChannel implements CommunicationChannel {
   dynamic noSuchMethod(Invocation invocation) {
     // Do nothing
+    return null;
   }
 }