Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/array_tracing_test.dart

Index: tests/lib/mirrors/array_tracing_test.dart
diff --git a/tests/lib/mirrors/array_tracing_test.dart b/tests/lib/mirrors/array_tracing_test.dart
index 092feacdc82c32d01118abc285f213946fa626c3..4845a66549a94a327268d3de01c91eb477416cdf 100644
--- a/tests/lib/mirrors/array_tracing_test.dart
+++ b/tests/lib/mirrors/array_tracing_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 {