Add @MirrorsUsed annotations to speed up tests.

tests/lib/mirrors/mirrors_nsm_test.dart

 // Copyright (c) 2011, 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 MirrorsTest;
 
+@MirrorsUsed(targets: "MirrorsTest")
 import 'dart:mirrors';
 import 'package:expect/expect.dart';
 
 bool isNSMContainingFieldName(e, String fieldName, bool isSetter) {
   print(e);
   if (e is! NoSuchMethodError) return false;
   String needle = fieldName;
   if (isSetter) needle += "=";
   return "$e".contains(needle) && ! "$e".contains(needle + "=");
 }
@@ skipping 94 matching lines @@
   expectMatchingErrors(() => instanceMirror.setField(#foo, null),
                        () => new A().foo= null);
   expectMatchingErrors(() => instanceMirror.setField(#finalInstance, null),
                        () => new A().finalInstance= null);
 }
 
 main() {
   testMessageContents();
   testMatchingMessages(); /// dart2js: ok
 }