How to add extra into text into flutter google map

问题:

The problem is how to infuse text overlap on the custom google map marker with text which represents the vehicle registration number.

I have tried to use this method to have the text overlay on the icon builder: (context) =>() But is not recognized at all.

  class MapsDemo extends StatefulWidget {
    @override
    State createState() => MapsDemoState();
  }

class MapsDemoState extends State<MapsDemo> {

  GoogleMapController mapController;
 //Map<PermissionGroup, PermissionStatus> permissions = await PermissionHandler().requestPermissions([PermissionGroup.contacts]);import 'package:permission_handler/permission_handler.dart';

  @override
  Widget build(BuildContext context) {
    return Scaffold(
          body: Column(
            children: <Widget>[
            Container(
                height: MediaQuery.of(context).size.height,
                width: MediaQuery.of(context).size.width,
                child: GoogleMap(
                 onMapCreated: (GoogleMapController controller) {
                   mapController = controller;
                 },
                ),
            ),
           ],
         ),
         floatingActionButton: FloatingActionButton(onPressed: () {

          double mq1 = MediaQuery.of(context).devicePixelRatio;
          String icon = "images/car.png";
          if (mq1>1.5 && mq1<2.5) {icon = "images/car2.png";}
          else if(mq1 >= 2.5){icon = "images/car3.png";}
          print("Mq 1"+mq1.toStringAsFixed(5));
          String iconPath="lib/assets/[e-mail]";


         mapController.addMarker(
              MarkerOptions(
                position: LatLng(37.4219999, -122.0862462),
                infoWindowText: InfoWindowText("TEST","TEST"),
                icon: BitmapDescriptor.fromAsset(iconPath),
                consumeTapEvents: true,

                /*builder: (context) =>(

                )*/

                //icon:BitmapDescriptor.fromAsset(assetName)
              ),
         );

         mapController.addMarker(
              MarkerOptions(
                position: LatLng(38.4219999, -122.0862462),
                infoWindowText: InfoWindowText("tt","adfaf"),
                icon: BitmapDescriptor.fromAsset("lib/assets/logo.png"),
                anchor: Offset(100,160),

                //icon:BitmapDescriptor.fromAsset(assetName)
              ),
         );

         mapController.animateCamera(
            CameraUpdate.newCameraPosition(
              CameraPosition(
                target: LatLng(37.4219999, -122.0862462),

                zoom: 15.0,
              ),
            ),
          );

        })
      );

  }

}

What I have shown is the icon appear correctly if you notice the white space on the right of the icon is where I want the registration number to appear.