Add top-level driver and abstract plugin superclass

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