Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/inherit_field_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.
 
 // Test inherited fields.
 
 library test.inherit_field_test;
 
+@MirrorsUsed(targets: "test.inherit_field_test")
 import 'dart:mirrors';
 
 import 'stringify.dart';
 
 class Foo {
   var field;
 }
 
 class Bar extends Foo {
 }
 
 void main() {
   expect('Variable(s(field) in s(Foo))',
          reflectClass(Foo).declarations[#field]);
   expect('<null>',
          reflectClass(Bar).declarations[#field]);
 }