Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/class_mirror_location_test.dart

Index: tests/lib/mirrors/class_mirror_location_test.dart
diff --git a/tests/lib/mirrors/class_mirror_location_test.dart b/tests/lib/mirrors/class_mirror_location_test.dart
index c6aa596e0b831abd74fecb97e5f32f696645911c..ee82c832c53e5c9933a5b18577f413a979eabedf 100644
--- a/tests/lib/mirrors/class_mirror_location_test.dart
+++ b/tests/lib/mirrors/class_mirror_location_test.dart
@@ -1,9 +1,9 @@
 // Copyright (c) 2015, 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.class_location;
 
+@MirrorsUsed(targets: "test.class_location")
 import "dart:mirrors";
 import "package:expect/expect.dart";