title: on-change event not firing properly when binding value
body: <p>I have this html code:</p>
  
  
  
  <pre class="lang-dart prettyprint-override"><code>&lt;textarea id="text-to-convert" on-change="change" value="{{text}}"&gt;&lt;/textarea&gt;
  </code></pre>
  
  <p>And this dart code:</p>
  
  <pre class="lang-dart prettyprint-override"><code>import "dart:html";
  import 'package:polymer/polymer.dart';
  
  @CustomTag('dictionary-converter')
  class DictionaryConverter extends PolymerElement with ObservableMixin {
  
    @observable String text = "Initial text";
  
    void change(Event event, var detail, TextAreaElement textElement) {
      print(textElement.value);
      print(text);
    }
  
  }
  </code></pre>
  
  <p>In this case, the <code>on-change</code> event is only triggered from time to time. (I haven't yet figured out when exactly).</p>
  
  <p>When I remove the <code>value={{text}}</code> binding, the event is properly fired every time the textare is changed.</p>
  
  <p>Am I overlooking something or is this a bug?</p>