anchor.diff:      Marker marker = new Marker(
           this, type, severity, message, lineNum, charStart, charEnd);
       _markers.add(marker);
  -    _fireMarkerEvent(new MarkerChangeEvent.fromSingle(new MarkerDelta(this, marker, EventType.ADD)));
  +    _fireMarkerEvent([new MarkerDelta(this, marker, EventType.ADD)]);
       return marker;
     }
   
  
positive.code: 
    void _fireResourceEvent(ResourceChangeEvent event) => _parent._fireResourceEvent(event);
  
    void _fireMarkerEvent(MarkerChangeEvent event) => _parent._fireMarkerEvent(event);
  
    Future delete();
  
  
negative.code: 
    List<MarkerDelta> changes;
  
    factory MarkerChangeEvent.fromList(List<MarkerDelta> deltas) {
      return new MarkerChangeEvent._(deltas.toList());
    }
  
  