anchor.diff:  part of domelement;
  +
  +typedef dynamic _Callback(DomElement target);
   
   abstract class ContentCapable {
     String get html => nativeElement.innerHtml;
  
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:       DomElement element =
            source is Element ? new DomElement.fromElement(source) : source;
        nativeElement.append(element.nativeElement);
      } else if (source is _Callback) {
        append(source(this));
      } else if (source != null) {
        throw new ArgumentError('Valid values are: ' +
            'String|Element|DomElement|(DomElement) => dynamic');
      }
    }
  
  