Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/generics_dynamic_test.dart

Index: tests/lib/mirrors/generics_dynamic_test.dart
diff --git a/tests/lib/mirrors/generics_dynamic_test.dart b/tests/lib/mirrors/generics_dynamic_test.dart
index b8abf534c2eff196513f697fe0d326ccbdd44879..6a4ec3ac938c981fc073c60b42594a4cab0bff68 100644
--- a/tests/lib/mirrors/generics_dynamic_test.dart
+++ b/tests/lib/mirrors/generics_dynamic_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';