anchor.diff:    @override
     attached() {
       this.template = this.querySelector('template');
  +    // Make sure they supplied a template in the content.
  +    if (this.template == null) {
  +      throw '\n\nIt looks like you are missing the <template> '
  +          'tag in your <core-list-dart> content. $CONTENT_ERROR';
  +    }
       if (templateBind(this.template).bindingDelegate == null) {
         templateBind(this.template).bindingDelegate = this.element.syntax;
       }
  
positive.code:     if (_physicalItems.length < _physicalCount) {
        _physicalItems.length = _physicalCount;
      }
      for (var i = 0, item = this.template.nextElementSibling;
           item != null && i < _physicalCount;
           ++i, item = item.nextElementSibling) {
        _physicalItems[i] = item;
        _transformValue[item] = 0;
      }
      this.refresh(true);
    }
  
  
negative.code: 
    @observable var selected;
  
    static const String CONTENT_ERROR = '\n\n'
  'The content of a <core-list-dart> element should be a single template tag '
  'which contains a single element (which can itself contain whatever content or '
  'elements you wish). For example: '
  '''\n\n
  <core-list-dart model="{{data}}">
    <template>
      <div>
        // All your custom content/elements here.
      </div>
    </template>
  </core-list-dart>
  ''';
  
    var _target;
    var _targetScrollSubscription;
    int _visibleCount;
  