Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/hot_set_field_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.
 
 library test.hot_set_field;
 
+@MirrorsUsed(targets: "test.hot_set_field")
 import 'dart:mirrors';
 import 'package:expect/expect.dart';
 
 class C {
   var field;
   var _field;
 }
 
 const int optimizationThreshold = 20;
 
@@ skipping 25 matching lines @@
   for (int i = 0; i < (2 * optimizationThreshold); i++) {
     Expect.throws(() => im.setField(selector, i), (e) => e is NoSuchMethodError);
   }
 }
 
 main() {
   testPublic();
   testPrivate();
   testPrivateWrongLibrary();
 }