SnackBarAction({Key key, String label, @required VoidCallback onPressed })

Creates an action for a SnackBar.

The label and onPressed arguments must be non-null.

Source 

SnackBarAction({
  Key key,
  this.label,
  @required this.onPressed
}) : super(key: key) {
  assert(label != null);
  assert(onPressed != null);
}