Creates a Positioned object with left, top, right, and bottom set to 0.0 unless a value for them is passed.
Source
Positioned.fill({
Key key,
Widget child,
this.left: 0.0,
this.top: 0.0,
this.right: 0.0,
this.bottom: 0.0
}) : width = null,
height = null,
super(key: key, child: child);