how to send push notification to particular user using fcm (firebase Messaging) and Flutter

var postUrl = "fcm.googleapis.com/fcm/send";

        static Future<void> sendNotification(receiver,msg)async{

        var token = await getToken(receiver);
        print('token : $token');

        final data = {
          "notification": {"body": "Accept Ride Request", "title": "This is Ride Request"},
          "priority": "high",
          "data": {
            "click_action": "FLUTTER_NOTIFICATION_CLICK",
            "id": "1",
            "status": "done"
          },
          "to": "$token"
        };

        final headers = {
          'content-type': 'application/json',
          'Authorization': 'key=<Your firebase Messaging Api Key Get it from firebase project settings under cloud messaging section>'
        };


        BaseOptions options = new BaseOptions(
          connectTimeout: 5000,
          receiveTimeout: 3000,
          headers: headers,
        );


        try {
          final response = await Dio(options).post(postUrl,
              data: data);

          if (response.statusCode == 200) {
            Fluttertoast.showToast(msg: 'Request Sent To Driver');
          } else {
            print('notification sending failed');
            // on failure do sth
          }
        }
        catch(e){
          print('exception $e');
        }




      }

      static Future<String> getToken(userId)async{

        final Firestore _db = Firestore.instance;

        var token;
        await _db.collection('users')
            .document(userId)
            .collection('tokens').getDocuments().then((snapshot){
              snapshot.documents.forEach((doc){
                token = doc.documentID;
              });
        });

        return token;


      }


    //Now Receiving End 

        class _LoginPageState extends State<LoginPage>
        with SingleTickerProviderStateMixin {

      final Firestore _db = Firestore.instance;
      final FirebaseMessaging _fcm = FirebaseMessaging();

      StreamSubscription iosSubscription;



    //this code will go inside intiState function 

    if (Platform.isIOS) {
          iosSubscription = _fcm.onIosSettingsRegistered.listen((data) {
            // save the token  OR subscribe to a topic here
          });

          _fcm.requestNotificationPermissions(IosNotificationSettings());
        }
        _fcm.configure(
          onMessage: (Map<String, dynamic> message) async {
            print("onMessage: $message");
            showDialog(
              context: context,
              builder: (context) => AlertDialog(
                content: ListTile(
                  title: Text(message['notification']['title']),
                  subtitle: Text(message['notification']['body']),
                ),
                actions: <Widget>[
                  FlatButton(
                    child: Text('Ok'),
                    onPressed: () => Navigator.of(context).pop(),
                  ),
                ],
              ),
            );
          },
          onLaunch: (Map<String, dynamic> message) async {
            print("onLaunch: $message");
            // TODO optional
          },
          onResume: (Map<String, dynamic> message) async {
            print("onResume: $message");
            // TODO optional
          },
        );

//saving token while signing in or signing up
 _saveDeviceToken(uid) async {
    FirebaseUser user = await _auth.currentUser();

    // Get the token for this device
    String fcmToken = await _fcm.getToken();

    // Save it to Firestore
    if (fcmToken != null) {
      var tokens = _db
          .collection('users')
          .document(uid)
          .collection('tokens')
          .document(fcmToken);

      await tokens.setData({
        'token': fcmToken,
        'createdAt': FieldValue.serverTimestamp(), // optional
        'platform': Platform.operatingSystem // optional
      });
    }
  }