anchor.diff:    void _clearMarkers({bool fireEvent : true}) {
       _markers.clear();
       if (fireEvent) {
  -      _fireMarkerEvent(new MarkerChangeEvent.fromSingle(new MarkerDelta(this, null, EventType.DELETE)));
  +      _fireMarkerEvent([new MarkerDelta(this, null, EventType.DELETE)]);
       }
     }
   
  
positive.code: 
    List<MarkerDelta> changes;
  
    factory MarkerChangeEvent.fromSingle(MarkerDelta delta) {
      return new MarkerChangeEvent._([delta]);
    }
  
    factory MarkerChangeEvent.fromList(List<MarkerDelta> deltas) {
      var event = new MarkerChangeEvent._(deltas.toList());
      return new MarkerChangeEvent._(deltas.toList());
    }
  
  
negative.code:   void clearMarkers() {
      traverse().where((r) => r.isFile)
          .forEach((f) => (f as File)._clearMarkers(fireEvent : false));
      _fireMarkerEvent([new MarkerDelta(this, null,EventType.DELETE)]);
    }
  
    int findMaxProblemSeverity(String type) {
  