An optional focus node to use as the focus node for this widget.
If one is not supplied, then one will be automatically allocated, owned,
and managed by this widget. The widget will be focusable even if a
focusNode is not supplied. If supplied, the given
focusNode will be
hosted by this widget, but not owned. See FocusNode for more
information on what being hosted and/or owned implies.
Supplying a focus node is sometimes useful if an ancestor to this widget wants to control when this widget has the focus. The owner will be responsible for calling FocusNode.dispose on the focus node when it is done with it, but this widget will attach/detach and reparent the node when needed.
final FocusNode focusNode