Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/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.
 
 library test.metadata_test;
 
+@MirrorsUsed(targets: "test.metadata_test")
 import 'dart:mirrors';
 
 const string = 'a metadata string';
 
 const symbol = const Symbol('symbol');
 
 const hest = 'hest';
 
 @symbol @string
 class MyClass {
@@ skipping 45 matching lines @@
   checkMetadata(method, []);
 
   VariableMirror xMirror = myClassMirror.declarations[#x];
   checkMetadata(xMirror, [hest, hest, symbol]);
 
   VariableMirror yMirror = myClassMirror.declarations[#y];
   checkMetadata(yMirror, []);
 
   // TODO(ahe): Test local functions.
 }