Clean up after some recent code changes

pkg/analyzer_plugin/test/plugin/navigation_mixin_test.dart

 // Copyright (c) 2017, 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.
 
 import 'dart:async';
 
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/file_system/memory_file_system.dart';
 import 'package:analyzer/src/dart/analysis/driver.dart';
 import 'package:analyzer_plugin/plugin/navigation_mixin.dart';
@@ skipping 46 matching lines @@
     expect(result, isNotNull);
     expect(result.files, hasLength(1));
     expect(result.targets, hasLength(1));
     expect(result.regions, hasLength(2));
   }
 
   test_sendNavigationNotification() async {
     await plugin.handleAnalysisSetContextRoots(
         new AnalysisSetContextRootsParams([contextRoot1]));
 
-    plugin.mockChannel.listen(null,
-        onNotification: (Notification notification) {
+    channel.listen(null, onNotification: (Notification notification) {
       expect(notification, isNotNull);
       AnalysisNavigationParams params =
           new AnalysisNavigationParams.fromNotification(notification);
       expect(params.files, hasLength(1));
       expect(params.targets, hasLength(1));
       expect(params.regions, hasLength(2));
     });
     await plugin.sendNavigationNotification(filePath1);
   }
 }
@@ skipping 27 matching lines @@
 
   @override
   Future<NavigationRequest> getNavigationRequest(
       AnalysisGetNavigationParams parameters) async {
     AnalysisResult result = new AnalysisResult(null, null, parameters.file,
         null, null, null, null, null, null, null, null);
     return new DartNavigationRequestImpl(
         resourceProvider, parameters.offset, parameters.length, result);
   }
 }