The data associated with a LongPressDraggable
is sent to a special widget called DragTarget,
where the user releases the drag gesture.
You’ll implement the drop behavior next.

Drop the draggable

The user can drop a LongPressDraggable wherever they choose,
but dropping the draggable has no effect unless it’s dropped
on top of a DragTarget. When the user drops a draggable on
top of a DragTarget widget, the DragTarget widget 
can either accept or reject the data from the draggable.

In this recipe, the user should drop a menu item on a
CustomerCart widget to add the menu item to the user’s cart.

Wrap the CustomerCart widget with a DragTarget widget.

The DragTarget displays your existing widget and
also coordinates with LongPressDraggable to recognize
when the user drags a draggable on top of the DragTarget.
The DragTarget also recognizes when the user drops
a draggable on top of the DragTarget widget.