Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/regress_26187_test.dart

Index: tests/lib/mirrors/regress_26187_test.dart
diff --git a/tests/lib/mirrors/regress_26187_test.dart b/tests/lib/mirrors/regress_26187_test.dart
index 1d99362e04433e11c9f3f6c1918c86f604e43e28..21dded51f8a0b93ae13ec33505cd6ab78dbf7dbf 100644
--- a/tests/lib/mirrors/regress_26187_test.dart
+++ b/tests/lib/mirrors/regress_26187_test.dart
@@ -2,6 +2,7 @@
 // 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.
 
+@MirrorsUsed(targets: const ["A", "B", "f", "g"])
 import 'dart:mirrors';
 import 'package:expect/expect.dart';