anchor.diff:  
     @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;
  
positive.code:   @override
    attached() {
      this.template = this.querySelector('template');
      if (templateBind(this.template).bindingDelegate == null) {
        templateBind(this.template).bindingDelegate = this.element.syntax;
      }
  
negative.code:     if (_physicalItems.length < _physicalCount) {
        _physicalItems.length = _physicalCount;
      }
      for (var i = 0, item = this.template.nextElementSibling; i < _physicalCount;
           ++i, item = item.nextElementSibling) {
        // Null item means they didn't have any elements in their <template>, only
        // text nodes (or a single binding most likely).
        if (item == null) {
          throw '\n\nIt looks like you are missing an element inside your '
              'template.$CONTENT_ERROR';
        }
        _physicalItems[i] = item;
        _transformValue[item] = 0;
      }
      // Check for multiple top level elements in a <template>
      if (_physicalItems.last.nextElementSibling != null) {
        throw '\n\n It looks like you have multiple top level elements inside '
            'your template. $CONTENT_ERROR';
      }
  
      this.refresh(true);
    }
  
  