Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/list_constructor_test.dart

Index: tests/lib/mirrors/list_constructor_test.dart
diff --git a/tests/lib/mirrors/list_constructor_test.dart b/tests/lib/mirrors/list_constructor_test.dart
index c25d038ac9d055fc89158c0c2e99a6b18fd8418e..ee584727d2a0efc1b10dda82ce8147212818f35d 100644
--- a/tests/lib/mirrors/list_constructor_test.dart
+++ b/tests/lib/mirrors/list_constructor_test.dart
@@ -2,8 +2,11 @@
 // 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;
+
 import "package:expect/expect.dart";
 
+@MirrorsUsed(targets: const ["lib", "dart.core"])
 import 'dart:mirrors';
 
 main() {