fix dart2js warnings in polymer_expressions pkg

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:mdv/mdv.dart' as mdv;
 import 'package:observe/observe.dart';
 import 'package:observe/src/microtask.dart';
 import 'package:polymer_expressions/polymer_expressions.dart';
 import 'package:unittest/html_config.dart';
 import 'package:unittest/unittest.dart';
 
 main() {
   mdv.initialize();
@@ skipping 57 matching lines @@
 
 class NotifyModel extends ChangeNotifierBase {
   var _x;
   NotifyModel([this._x]);
 
   get x => _x;
   set x(value) {
     _x = notifyPropertyChange(const Symbol('x'), _x, value);
   }
 }