Add tests for recently added mixins and improve plugin tests

pkg/analyzer_plugin/test/plugin/test_all.dart

Index: pkg/analyzer_plugin/test/plugin/test_all.dart
diff --git a/pkg/analyzer_plugin/test/plugin/test_all.dart b/pkg/analyzer_plugin/test/plugin/test_all.dart
index c727a3612c553bfa5208ca1dd63038f3c88573a1..1b61afb6e6decda99ab003cb9dc33aabd26132f4 100644
--- a/pkg/analyzer_plugin/test/plugin/test_all.dart
+++ b/pkg/analyzer_plugin/test/plugin/test_all.dart
@@ -4,10 +4,18 @@
 
 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');
 }