reverse property
Setting reverse to true places header at the opposite end of the
sliver than normal. This is most useful when a SliverStickyHeader is
above ScrollView.center and you want to place the header at the top.
Above the center slivers grow in the opposite direction and this option
counteracts this behaviour. When reverse is false (default) and
ScrollView.reverse is false, headers in slivers above
ScrollView.center appear at the bottom.
Implementation
final bool reverse;