FittedBox({Key key, ImageFit fit: ImageFit.contain, FractionalOffset alignment: FractionalOffset.center, Widget child })

Creates a widget that scales and positions its child within itself according to fit.

The fit and alignment arguments must not be null.

Source 

FittedBox({
  Key key,
  this.fit: ImageFit.contain,
  this.alignment: FractionalOffset.center,
  Widget child
}) : super(key: key, child: child) {
  assert(fit != null);
  assert(alignment != null && alignment.dx != null && alignment.dy != null);
}