ifSync method
Performs a side-effect if this is Sync.
Implementation
@override
@pragma('vm:prefer-inline')
Sync<T> ifSync(
  @noFutures void Function(Sync<T> self, Sync<T> sync) noFutures,
) {
  return Sync(() {
    noFutures(this, this);
    return this;
  }).flatten().sync().unwrap();
}