Clean up after some recent code changes

pkg/analyzer_plugin/test/plugin/navigation_mixin_test.dart

Index: pkg/analyzer_plugin/test/plugin/navigation_mixin_test.dart
diff --git a/pkg/analyzer_plugin/test/plugin/navigation_mixin_test.dart b/pkg/analyzer_plugin/test/plugin/navigation_mixin_test.dart
index eea7c012a16e743539e855b7610be793ec1ade60..5be3c912c43202b676a6f48d255c5239046b424e 100644
--- a/pkg/analyzer_plugin/test/plugin/navigation_mixin_test.dart
+++ b/pkg/analyzer_plugin/test/plugin/navigation_mixin_test.dart
@@ -64,8 +64,7 @@ class NavigationMixinTest {
     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);