this function is all your need for screenshot a widget and save locally:
static GlobalKey _repaintKey = GlobalKey();
Widget _yourWidget(){
  return Stack(
    key: _repaintKey,
    children: [
    ],
  );
}
Future<void> _takeScreenShot(context) async {
    RenderRepaintBoundary boundary = _repaintKey.currentContext.findRenderObject();
    ui.Image image = await boundary.toImage();
    ByteData byteData = await image.toByteData(format: ui.ImageByteFormat.png);
    Uint8List pngBytes = byteData.buffer.asUint8List();
    final path = join(
      (await getTemporaryDirectory()).path, "screenshot${DateTime.now().toIso8601String()}.png");
    File imgFile = File(path);
    imgFile.writeAsBytes(pngBytes).then((value) {
      Navigator.of(context).pushNamed(Routes.uploadImage, arguments: value.uri.path);
    }).catchError((onError) {
      print(onError);
    });
}