Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/metadata_constructor_arguments_test.dart

 // Copyright (c) 2013, 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 13817.
 
 library test.metadata_constructor_arguments;
 
+@MirrorsUsed(targets: "test.metadata_constructor_arguments")
 import 'dart:mirrors';
 import 'package:expect/expect.dart';
 
 class Tag {
   final name;
   const Tag({named}) : this.name = named;
 }
 
 @Tag(named: undefined)  /// 01: compile-time error
 class A {}
@@ skipping 44 matching lines @@
   reflectClass(A).metadata;
   checkMetadata(reflectClass(B), [const Tag(named: 'valid')]);
   checkMetadata(reflectClass(C), [const Tag(named: C.STATIC_FIELD)]);
   reflectClass(D).metadata;
   reflectClass(E).metadata;
   reflectClass(F).metadata;
   reflectClass(G).metadata;
   reflectClass(H).metadata;
   reflectClass(I).metadata;
 }