Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/generic_superclass_test.dart

Index: tests/lib/mirrors/generic_superclass_test.dart
diff --git a/tests/lib/mirrors/generic_superclass_test.dart b/tests/lib/mirrors/generic_superclass_test.dart
index ebf4c8cf96b22f8f52474a23933b036a47aec6a7..ca6a5d7c10f5ad8160b29ee063e55d50ad5a8981 100644
--- a/tests/lib/mirrors/generic_superclass_test.dart
+++ b/tests/lib/mirrors/generic_superclass_test.dart
@@ -2,7 +2,11 @@
 // 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;
+
 import 'package:expect/expect.dart';
+
+@MirrorsUsed(targets: "lib")
 import 'dart:mirrors';