Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/mixin_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.mixin;
 
+@MirrorsUsed(targets: "test.mixin")
 import 'dart:mirrors';
 
 import 'package:expect/expect.dart';
 
 class Super {}
 class Mixin {}
 class Mixin2 {}
 class Mixin3 {}
 
 class MixinApplication = Super with Mixin;
@@ skipping 19 matching lines @@
                 reflectClass(MultipleMixins).superclass.mixin);
   Expect.equals(reflectClass(Mixin2),
                 reflectClass(MultipleMixins).superclass.superclass.mixin);
   Expect.equals(reflectClass(Mixin),
                 reflectClass(MultipleMixins).superclass.superclass.superclass
                     .mixin);
   Expect.equals(reflectClass(Super),
                 reflectClass(MultipleMixins).superclass.superclass.superclass
                     .superclass.mixin);
 }