Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/method_mirror_returntype_test.dart

Index: tests/lib/mirrors/method_mirror_returntype_test.dart
diff --git a/tests/lib/mirrors/method_mirror_returntype_test.dart b/tests/lib/mirrors/method_mirror_returntype_test.dart
index 42ccda287ff985cc593a7c66fa8e1f6042b3bd1d..8f574586b79320c9eed8c1dfb05e940ca38841a0 100644
--- a/tests/lib/mirrors/method_mirror_returntype_test.dart
+++ b/tests/lib/mirrors/method_mirror_returntype_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";