Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/metadata_class_mirror_test.dart

 // Copyright (c) 2014, 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 http://dartbug.com/19173
 
+library lib;
+
+@MirrorsUsed(targets: "lib")
 import 'dart:mirrors';
 
 class A {
     const A();
 }
 
 @deprecated
 const A anA = const A();
 
 main() {
   ClassMirror typeMirror = reflectType(A);
   var decs = typeMirror.declarations;
   print(decs.length);
 }