DecoratedBox({Key key, @required Decoration decoration, DecorationPosition position: DecorationPosition.background, Widget child })

Creates a widget that paints a Decoration.

The decoration and position arguments must not be null. By default the decoration paints behind the child.

Source 

DecoratedBox({
  Key key,
  @required this.decoration,
  this.position: DecorationPosition.background,
  Widget child
}) : super(key: key, child: child) {
  assert(decoration != null);
  assert(position != null);
}