Add tests for recently added mixins and improve plugin tests

pkg/analyzer_plugin/test/plugin/test_all.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 'package:test_reflective_loader/test_reflective_loader.dart';
 
+import 'assist_mixin_test.dart' as assist_mixin_test;
+import 'completion_mixin_test.dart' as completion_mixin_test;
+import 'fix_mixin_test.dart' as fix_mixin_test;
+import 'navigation_mixin_test.dart' as navigation_mixin_test;
 import 'plugin_test.dart' as plugin_test;
 
 main() {
   defineReflectiveSuite(() {
+    assist_mixin_test.main();
+    completion_mixin_test.main();
+    fix_mixin_test.main();
+    navigation_mixin_test.main();
     plugin_test.main();
   }, name: 'plugin');
 }