Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/no_metadata_test.dart

Index: tests/lib/mirrors/no_metadata_test.dart
diff --git a/tests/lib/mirrors/no_metadata_test.dart b/tests/lib/mirrors/no_metadata_test.dart
index f5c0747a59928a9d4a8c29bc82bedf659940c368..062f9bcee6fcb5bc5e66e63e1e73aec17c2ffa6e 100644
--- a/tests/lib/mirrors/no_metadata_test.dart
+++ b/tests/lib/mirrors/no_metadata_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 'stringify.dart';