bellWithUnreadCount property
Label for the bell while it shows an unread badge. Include countPlaceholder where the number of unread messages should appear; it is substituted natively at render time. null → falls back to bell.
The badge itself is always hidden from assistive technologies, so the count is announced only through this label, never as bare digits appended to the button.
This is a template rather than a callback because configuration crosses the platform channel, which carries data but not functions. One template cannot express languages whose plural rules need a distinct form per count.
Implementation
final String? bellWithUnreadCount;