RenderStack({List<RenderBox> children, FractionalOffset alignment: FractionalOffset.topLeft, Overflow overflow: Overflow.clip })

Creates a stack render object.

By default, the non-positioned children of the stack are aligned by their top left corners.

Source 

RenderStack({
  List<RenderBox> children,
  FractionalOffset alignment: FractionalOffset.topLeft,
  Overflow overflow: Overflow.clip
}) : _alignment = alignment,
     _overflow = overflow {
  assert(overflow != null);
  addAll(children);
}