Start-aligned FloatingActionButton, floating over the transition between
the Scaffold.appBar and the Scaffold.body.
To align a floating action button with CircleAvatars in the
ListTile.leading slots of ListTiles in a ListView in the Scaffold.body,
use miniStartTop and set FloatingActionButton.mini to true.
This is unlikely to be a useful location for apps that lack a top AppBar
or that use a SliverAppBar in the scaffold body itself.
static const FloatingActionButtonLocation startTop = _StartTopFabLocation();