Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/redirecting_factory_test.dart

Index: tests/lib/mirrors/redirecting_factory_test.dart
diff --git a/tests/lib/mirrors/redirecting_factory_test.dart b/tests/lib/mirrors/redirecting_factory_test.dart
index 6f0ddf2fbbdf9201a5412d0663ca6b1dddb23e66..5cd0297850e4ec2f99945badd7a024acc0686743 100644
--- a/tests/lib/mirrors/redirecting_factory_test.dart
+++ b/tests/lib/mirrors/redirecting_factory_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";