Implement more of the behavior of a plugin; refactor some API

pkg/analyzer_plugin/test/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 'plugin/test_all.dart' as plugin;
 import 'src/test_all.dart' as src;
 import 'utilities/test_all.dart' as utilities;
 
 main() {
   defineReflectiveSuite(() {
+    plugin.main();
     src.main();
     utilities.main();
   }, name: 'analyzer_plugin');
 }