Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/closures_test.dart

Index: tests/lib/mirrors/closures_test.dart
diff --git a/tests/lib/mirrors/closures_test.dart b/tests/lib/mirrors/closures_test.dart
index 35e81f9805821b2bd2becc4aa986948e3379bfc0..858e14e6f4faaeec6c601ff6655439d2a267b8ca 100644
--- a/tests/lib/mirrors/closures_test.dart
+++ b/tests/lib/mirrors/closures_test.dart
@@ -2,6 +2,9 @@
 // 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 lib;
+
+@MirrorsUsed(targets: "lib")
 import 'dart:mirrors';
 import 'package:expect/expect.dart';
 import 'stringify.dart';