Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/regress_16321_test.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // 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.
 
 // Regression test for Issue 16321.
 // (Type errors in metadata crashed the VM in checked mode).
 
+library lib;
+
+@MirrorsUsed(targets: "lib")
 import "dart:mirrors";
 
 class TypedBox {
   final List<String> contents;
   const TypedBox(this.contents);
 }
 
 @TypedBox('foo')  /// 01: static type warning, checked mode compile-time error
 @TypedBox(const ['foo'])
 class C {}
 
 main() {
   reflectClass(C).metadata;
 }