anchor.diff:        List<Node> childNodes = nativeElement.childNodes;
         Node firstChild = childNodes.length > 0 ? childNodes.first : null;
         nativeElement.insertBefore(element.nativeElement, firstChild);
  -    } else {
  +    } else if (source is _Callback) {
  +      prepend(source(this));
  +    } else if (source != null) {
         throw new ArgumentError('Valid values are: String|Element|DomElement');
       }
     }
  
positive.code:     nativeElement.text = new HtmlEscape().convert(value);
    }
  
    void append(dynamic /* String|Element|DomElement */ source) {
      if (source is String) {
        nativeElement.insertAdjacentHtml('beforeend', source,
            treeSanitizer: new NullTreeSanitizer());
  
negative.code: part of domelement;
  
  typedef dynamic _Callback(DomElement target);
  
  abstract class ContentCapable {
    String get html => nativeElement.innerHtml;
  