Adding @MirrorsUsed to html/custom/mirrors test

tests/html/custom/mirrors_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 tests.html.mirrors_test;
 
+@MirrorsUsed(targets: const [A, B])
 import 'dart:mirrors';
 import 'dart:html';
 import 'package:unittest/unittest.dart';
 import 'package:unittest/html_config.dart';
 import '../utils.dart';
 
 /// Regression test for a tricky mirrors+custom_elements issue:
 /// dart2js mirrors cache dispatch information on the Object's constructor.
 /// This was failing for custom elements on IE 10, because the constructor was
 /// HTMLUnknownElement for all of them. So mirrors called the wrong method.
@@ skipping 40 matching lines @@
 
   int _fooBar = 2;
   int _fooBarSet = 0;
 
   int get fooBar => _fooBar;
   set fooBar(value) {
     _fooBarSet++;
     _fooBar = value;
   }
 }