Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/generic_list_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.superclass;
 
+@MirrorsUsed(targets: const ["test.superclass", "dart.core"])
 import 'dart:mirrors';
 import 'package:expect/expect.dart';
 
 class Foo<T> {
   List<T> makeList() {
     if (new DateTime.now().millisecondsSinceEpoch == 42) return [];
     return new List<T>();
   }
 }
 
 main() {
   List<String> list = new Foo<String>().makeList();
   var cls = reflectClass(list.runtimeType);
   Expect.isNotNull(cls, 'Failed to reflect on MyClass.');
 }