Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/null2_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.null_test;
 
+@MirrorsUsed(targets: const ["test.null_test", Null])
 import 'dart:mirrors';
 
 import 'package:expect/expect.dart';
 
 main() {
   InstanceMirror nullMirror = reflect(null);
   for (int i = 0; i < 10; i++) {
     Expect.isTrue(nullMirror.getField(#hashCode).reflectee is int);
   }
 }