Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/abstract_test.dart

 // Copyright (c) 2013, 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 abstract classes are retained.
 
 library test.abstract_test;
 
+@MirrorsUsed(targets: "test.abstract_test")
 import 'dart:mirrors';
 
 import 'stringify.dart';
 
 abstract class Foo {
 }
 
 void main() {
   expect('Class(s(Foo) in s(test.abstract_test), top-level)',
          reflectClass(Foo));
 }