Added back helper file accidentally deleted during test block 219 migration.

tests/lib_2/mirrors/deferred_mirrors_metatarget_test.dart

 // Copyright (c) 2014, 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.
 
 /// Test that metaTargets can be reached via the mirrorSystem.
 
 import 'package:async_helper/async_helper.dart';
 import 'package:expect/expect.dart';
 
 import "deferred_mirrors_metatarget_lib.dart" deferred as lib;
 
 void main() {
   asyncStart();
   lib.loadLibrary().then((_) {
     Expect.equals("A", lib.foo());
     asyncEnd();
   });
 }