saveToFile method
存储到文件中去 >>>
Implementation
Future saveToFile(String filePath) async {
ui.Image image = await _toImage();
var bytes = await (image.toByteData(format: ui.ImageByteFormat.png)
as FutureOr<ByteData>);
Uint8List ul = bytes.buffer.asUint8List();
File file = File(filePath);
file.openWrite();
file.writeAsBytesSync(ul);
}