Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/generic_class_declaration_test.dart

Index: tests/lib/mirrors/generic_class_declaration_test.dart
diff --git a/tests/lib/mirrors/generic_class_declaration_test.dart b/tests/lib/mirrors/generic_class_declaration_test.dart
index 0b8260f9f24332772b060e8af4fa4c0d58798da1..88d76b3c815bc9a2fcf06b1bba64ba778e4e4908 100644
--- a/tests/lib/mirrors/generic_class_declaration_test.dart
+++ b/tests/lib/mirrors/generic_class_declaration_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';