anchor.diff:        key: _scaffoldKey,
         appBar: new AppBar(title: new Text('Persistent bottom sheet')),
         floatingActionButton: new FloatingActionButton(
  -        onPressed: null,
  +        onPressed: _showMessage,
           backgroundColor: Colors.redAccent[200],
           child: new Icon(Icons.add)
         ),
  
positive.code:     })
      .closed.then((_) {
        setState(() { // re-enable the button
          _showBottomSheetCallback = showBottomSheet;
        });
      });
    }
  
    @override
  
negative.code:   @override
    void initState() {
      super.initState();
      _showBottomSheetCallback = _showBottomSheet;
    }
  
  
    void _showBottomSheet() {
      setState(() { // disable the button
        _showBottomSheetCallback = null;
      });
  