Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/mirrors_resolve_fields_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.
 
 // Regression test for dart2js that used to not resolve instance
 // fields when a class is only instantiated through mirrors.
 
+library lib;
+
 import "package:expect/expect.dart";
+
+@MirrorsUsed(targets: "lib")
 import 'dart:mirrors';
 
 class A {
   static const int _STATE_INITIAL = 0;
   int _state = _STATE_INITIAL;
   A();
 }
 
 main() {
   var mirrors = currentMirrorSystem();  
   var classMirror = reflectClass(A);
   var instanceMirror = classMirror.newInstance(const Symbol(''), []);
   Expect.equals(A._STATE_INITIAL, instanceMirror.reflectee._state);
 }