Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/superclass_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.superclass;
 
+@MirrorsUsed(targets: "test.superclass")
 import 'dart:mirrors';
 import 'package:expect/expect.dart';
 
 class MyClass {
 }
 
 main() {
   var cls = reflectClass(MyClass);
   Expect.isNotNull(cls, 'Failed to reflect on MyClass.');
   var superclass = cls.superclass;
   Expect.isNotNull(superclass, 'Failed to obtain superclass of MyClass.');
   Expect.equals(
       reflectClass(Object), superclass, 'Superclass of MyClass is not Object.');
   Expect.isNull(superclass.superclass, 'Superclass of Object is not null.');
 }