Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/unmangled_type_test.dart

Index: tests/lib/mirrors/unmangled_type_test.dart
diff --git a/tests/lib/mirrors/unmangled_type_test.dart b/tests/lib/mirrors/unmangled_type_test.dart
index dbc6b62437908677c6fb6c1dab670b8d0dd7d2b6..835adcdf0a516cab7a0ac40215f3d04fba730a19 100644
--- a/tests/lib/mirrors/unmangled_type_test.dart
+++ b/tests/lib/mirrors/unmangled_type_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';