Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/repeated_private_anon_mixin_app_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.
 
 library test.repeated_private_anon_mixin_app;
 
 // Regression test for symbol mangling.
 
+@MirrorsUsed(targets: "test.repeated_private_anon_mixin_app")
 import 'dart:mirrors';
 import 'package:expect/expect.dart';
 
 import 'repeated_private_anon_mixin_app1.dart' as lib1;
 import 'repeated_private_anon_mixin_app2.dart' as lib2;
 
 testMA() {
   Symbol name1 = reflectClass(lib1.MA).superclass.simpleName;
   Symbol name2 = reflectClass(lib2.MA).superclass.simpleName;
 
@@ skipping 12 matching lines @@
   Expect.equals('lib._S with lib._M, lib._M2', MirrorSystem.getName(name2));
 
   Expect.notEquals(name1, name2);
   Expect.notEquals(name2, name1);
 }
 
 main() {
   testMA();
   testMA2();
 }