anchor.diff:  import 'dart:html';
   
   Logger _log = new Logger('slick.dnd');
  -
  +const String _DND_TYPE='text';
   class DragAndDrop {
     Element rootEl;
     Element _dragSourceEl;
  
positive.code:       _log.finest('trigger resort column');
  
        List cols = _grid.getColumns();
        Column srcCol = cols[_grid.columnsById[event.dataTransfer.getData('text/html')]];
        Column destCol = cols[_grid.columnsById[headerElem.dataset['id']]];
        int srcIdx = cols.indexOf(srcCol);
        int destIdx = cols.indexOf(destCol);
  
negative.code:     event.dataTransfer.effectAllowed = 'move';
      //event.dataTransfer.setDragImage(ghost, 0, 0);
      //event.dataTransfer.setData("source_id", headerElem.dataset['id']);
      event.dataTransfer.setData(_DND_TYPE, headerElem.dataset['id']);
      // event.dataTransfer.setData('text/html', dragTarget.innerHtml);
    }
  
  