SliverOverlapAbsorber constructor
Creates a sliver that absorbs overlap and reports it to a SliverOverlapAbsorberHandle.
The handle must not be null.
The child must be a sliver.
Implementation
const SliverOverlapAbsorber({
  Key key,
  @required this.handle,
  Widget child,
}) : assert(handle != null),
     super(key: key, child: child);