Polymer core selector is not updated

The Dart code for my Polymer element looks like this:

@CustomTag('my-element')
class MyElement extends PolymerElement {
  final List<String> colors = toObservable(['red', 'green', 'blue']);
  MyElement.created() : super.created();
}

      

        
        
        
      

    

And the HTML looks like this:

<polymer-element name="my-element">
  <template>
    <style>
      .core-selected {
        font-weight: bold;
      }
    </style>
    <core-selector id="selector" selected="1">
      <template repeat="{{color in colors}}">
        <div value="{{color}}">{{color}}</div>
      </template>
    </core-selector>
    <hr>
    <!-- Prints the selected index, but does not update -->
    <div>{{$['selector'].selected]}}</div> 
  </template>
  <script type="application/dart" src="my_element.dart"></script>
</polymer-element>

      

        
        
        
      

    

Using <div>{{$['selector'].selected]}}</div>

correctly displays the index of the selected one color

, but choosing a different color does not update the value selected

. Am I using this correctly? Or is it a mistake?