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

tests/lib/mirrors/deferred_mirrors_metadata_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.
-
-@A(const B())
-library main;
-
-@B()
-import 'package:async_helper/async_helper.dart';
-import 'package:expect/expect.dart';
-
-import "dart:math";
-
-import 'deferred_mirrors_metadata_lib.dart' deferred as lib1;
-
-class A {
-  final B b;
-  const A(this.b);
-  String toString() => "A";
-}
-
-class B {
-  const B();
-  String toString() => "B";
-}
-
-class C {
-  const C();
-  String toString() => "C";
-}
-
-class D {
-  const D();
-  String toString() => "D";
-}
-
-void main() {
-  asyncStart();
-  lib1.loadLibrary().then((_) {
-    Expect.equals("ABCD", lib1.foo());
-    new C();
-    new D();
-    asyncEnd();
-  });
-}