Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/library_metadata_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.
 
 @string @symbol
 library test.library_metadata_test;
 
+@MirrorsUsed(targets: "test.library_metadata_test")
 import 'dart:mirrors';
 
 import 'metadata_test.dart';
 
 main() {
   MirrorSystem mirrors = currentMirrorSystem();
   checkMetadata(mirrors.findLibrary(#test.library_metadata_test),
                 [string, symbol]);
   checkMetadata(mirrors.findLibrary(#test.metadata_test), []);
 }