Fix test timeouts. I forgot to include observe/mirrors_used.dart in tests from

pkg/polymer_expressions/test/bindings_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 bindings_test;
 
 import 'dart:async';
 import 'dart:html';
 
 import 'package:logging/logging.dart';
 import 'package:observe/observe.dart';
+import 'package:observe/mirrors_used.dart'; // make test smaller.
 import 'package:observe/src/dirty_check.dart' show dirtyCheckZone;
 import 'package:polymer_expressions/polymer_expressions.dart';
 import 'package:template_binding/template_binding.dart' show templateBind;
 import 'package:unittest/html_config.dart';
 import 'package:unittest/unittest.dart';
 
 main() => dirtyCheckZone().run(() {
   useHtmlConfiguration();
 
   group('bindings', () {
@@ skipping 104 matching lines @@
 @reflectable
 class NotifyModel extends ChangeNotifier {
   var _x;
   NotifyModel([this._x]);
 
   get x => _x;
   set x(value) {
     _x = notifyPropertyChange(#x, _x, value);
   }
 }