file method Null safety

Future<File> file()

Implementation

@JSName('file')
Future<File> file() {
  var completer = new Completer<File>();
  _file((value) {
    applyExtension('File', value);
    completer.complete(value);
  }, (error) {
    completer.completeError(error);
  });
  return completer.future;
}