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

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