Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/spawn_function_root_library_test.dart

Index: tests/lib/mirrors/spawn_function_root_library_test.dart
diff --git a/tests/lib/mirrors/spawn_function_root_library_test.dart b/tests/lib/mirrors/spawn_function_root_library_test.dart
index 89b9475ef40a2b607d8a39a9f52ec887576041b0..4bf59dcff11485414973ef08b48205025038da5d 100644
--- a/tests/lib/mirrors/spawn_function_root_library_test.dart
+++ b/tests/lib/mirrors/spawn_function_root_library_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 'dart:isolate';
 import 'package:expect/expect.dart';