Candidate(DropController dropController) {
   this.dropController = dropController;
   Widget target = dropController.getDropTarget();
   if (!target.isAttached()) {
     throw new IllegalStateException(
         "Unattached drop target. You must call DragController#unregisterDropController for all drop targets not attached to the DOM.");
   }
   targetArea = new WidgetArea(target, null);
 }
 Widget getDropTarget() {
   return dropController.getDropTarget();
 }