Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/intercepted_cache_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.
 
 // This is a test for a problem in how dart2js cached InstanceMirror.invoke,
 // etc. The test is using getField, as invoke, setField, and getField all share
 // the same caching.
 
+library lib;
+
+@MirrorsUsed(targets: const ["lib", "dart.core"])
 import 'dart:mirrors';
 
 import 'package:expect/expect.dart';
 
 
 class Foo {
   Foo(this.length);
   int length;
 }
 
 main() {
   Expect.equals(
       1, reflect(new Foo(1)).getField(#length).reflectee);
   Expect.equals(
       2, reflect(new Foo(2)).getField(#length).reflectee);
   Expect.equals(0, reflect([]).getField(#length).reflectee);
 }