IndexedStack constructor
Creates a Stack widget that paints a single child.
The index argument must not be null.
Implementation
IndexedStack({
  Key key,
  AlignmentGeometry alignment = AlignmentDirectional.topStart,
  TextDirection textDirection,
  StackFit sizing = StackFit.loose,
  this.index = 0,
  List<Widget> children = const <Widget>[],
}) : super(key: key, alignment: alignment, textDirection: textDirection, fit: sizing, children: children);