visibleMessage method
The message messageId when principalId sees its thread; else 404 not_found.
Implementation
JsonMap visibleMessage(String principalId, String messageId) {
final m = messages[messageId];
if (m == null || !seesThread(principalId, m['thread_id']! as String)) {
throw const FakeProblem(404, 'not_found');
}
return m;
}