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