Positioned.fill constructor

const Positioned.fill({Key key, double left: 0.0, double top: 0.0, double right: 0.0, double bottom: 0.0, @required Widget child })

Creates a Positioned object with left, top, right, and bottom set to 0.0 unless a value for them is passed.

Implementation 

const Positioned.fill({
  Key key,
  this.left = 0.0,
  this.top = 0.0,
  this.right = 0.0,
  this.bottom = 0.0,
  @required Widget child,
}) : width = null,
     height = null,
     super(key: key, child: child);