Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/array_tracing2_test.dart

Index: tests/lib/mirrors/array_tracing2_test.dart
diff --git a/tests/lib/mirrors/array_tracing2_test.dart b/tests/lib/mirrors/array_tracing2_test.dart
index 39411a10b77846901fde1642dc57c9151f13aad6..cd7c9058f4b4d732e77ef177575de056f9f31e8e 100644
--- a/tests/lib/mirrors/array_tracing2_test.dart
+++ b/tests/lib/mirrors/array_tracing2_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: const ["lib", "dart.core"])
 import 'dart:mirrors';
 
 class A {