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

tests/lib_2/mirrors/deferred_mirrors_metatarget_lib.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.
 
 library lib;
 
 @MirrorsUsed(metaTargets: "MetaTarget")
 import "dart:mirrors";
 
 class MetaTarget {
   const MetaTarget();
 }
 
 @MetaTarget()
 class A {
   String toString() => "A";
 }
 
 String foo() {
   ClassMirror a = currentMirrorSystem().findLibrary(#lib).declarations[#A];
   return a.newInstance(const Symbol(""), []).invoke(#toString, []).reflectee;
 }