toJson method
Implementation
@override
Map<String, dynamic> toJson([dynamic extra]) {
  return {
    "@type": CONSTRUCTOR,
    "origin": origin.toJson(),
    "date": date,
    "public_service_announcement_type": publicServiceAnnouncementType,
    "from_chat_id": fromChatId,
    "from_message_id": fromMessageId,
  };
}