send_push_notification_dart/send_push_notification.dart

/// Packages used:
///   firebase_messaging
///   http
///
/// Send push notification to device by [token]
///
/// Get your token:
///   await FirebaseMessaging.instance.getToken();
Future<void> testFirebaseMessaging(String token) async {
  log('Testing cloud messaging');
  
  Map<String, String> headers = {
    HttpHeaders.authorizationHeader:
        'key = YOUR SERVER KEY',
    HttpHeaders.contentTypeHeader: "application/json"
  };

  Map<String, dynamic> msg = {'mesgType': 'ACTION', 'action_id': '4664'};

  Map<String, dynamic> notification = {
    'title': 'Title',
    'body': 'body text',
    'sound': 'default'
  };
  Map<String, dynamic> args = {
    'registration_ids': [token],
    'data': msg,
    'notification': notification
  };

  final res = await post(
    Uri.parse('https://fcm.googleapis.com/fcm/send'),
    headers: headers,
    body: json.encode(args),
  );
  log('Code: ${res.statusCode} Body: ${res.body}');
}