onDragStart method
Implementation
void onDragStart(DragStartDetails details) {
// It's possible for _hold to become null between _handleDragDown and
// _handleDragStart, for example if some user code calls jumpTo or otherwise
// triggers a new activity to begin.
assert(_drag == null);
_drag = position.drag(details, _disposeDrag);
assert(_drag != null);
assert(_hold == null);
}