Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/metadata_constructed_constant_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.metadata_constructed_constant_test;
 
+@MirrorsUsed(targets: "test.metadata_constructed_constant_test")
 import 'dart:mirrors';
 
 import 'package:expect/expect.dart';
 
 class ConstructedConstant {
   final value;
   const ConstructedConstant(this.value);
   toString() => 'ConstructedConstant($value)';
 }
 
 class Foo {
   @ConstructedConstant(StateError)
   m() {}
 }
 
 main() {
   var value =
       reflectClass(Foo).declarations[#m].metadata.single.reflectee;
   Expect.stringEquals('ConstructedConstant($StateError)', '$value');
 }