The shape of the snack bar's Material.
Defines the snack bar's Material.shape.
If this property is null then
ThemeData.snackBarTheme.shape is used.
If that's null then the shape will depend on the SnackBarBehavior. For
SnackBarBehavior.fixed, no overriding shape is specified, so the
SnackBar is rectangular. For SnackBarBehavior.floating, it uses a
RoundedRectangleBorder with a circular corner radius of 4.0.
final ShapeBorder shape