Fix two-way binding in polymer expressions.

pkg/polymer/test/event_handlers_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 polymer.test.event_handlers_test;
 
 import 'dart:async';
 import 'dart:html';
 
 import 'package:polymer/polymer.dart';
 import 'package:template_binding/template_binding.dart';
 import 'package:unittest/unittest.dart';
 import 'package:unittest/html_config.dart';
 
 @CustomTag('x-test')
 class XTest extends PolymerElement {
   int _testCount = 0;
   String _lastEvent;
   String _lastMessage;
-  List list1 = [];
-  List list2 = [];
+  List list1 = toObservable([]);
+  List list2 = toObservable([]);
   Future _onTestDone;
 
   XTest.created() : super.created();
 
   ready() {
     super.ready();
     for (var i = 0; i < 10; i++) {
       var model = new MiniModel(this, i);
       list1.add(model);
       list2.add(model);
@@ skipping 67 matching lines @@
 @initMethod main() {
   useHtmlConfiguration();
 
   setUp(() => Polymer.onReady);
   test('events handled', () {
     XTest test = querySelector('x-test');
     expect(test._onTestDone, isNotNull, reason: 'ready was called');
     return test._onTestDone;
   });
 }