// ---------- you can use
Uint8List? customMarker;

Future<Uint8List> changeIconLocation() async {
    ByteData byteData = await DefaultAssetBundle.of(context).load("assets/img/icon/category_2.png");
    customMarker = byteData.buffer.asUint8List();
    return byteData.buffer.asUint8List();
  }

Marker(
    icon: BitmapDescriptor.fromBytes(customMarker!,size: const Size(100,100)),
    );

// ----------- Or
  BitmapDescriptor sourceIcon = BitmapDescriptor.defaultMarker;

  void setCustomMarker() {
      BitmapDescriptor.fromAssetImage(
        ImageConfiguration.empty,
        'assets/img/map/marker-5.png',
      ).then((onValue) {
        destinationIcon = onValue;
      });
    }

	Marker(
    	icon: sourceIcon
      );

// ----------- Or


  Future<BitmapDescriptor> _getAssetIcon(BuildContext context, String icon) async {
    final Completer<BitmapDescriptor> bitmapIcon = Completer<BitmapDescriptor>();
    final ImageConfiguration config = createLocalImageConfiguration(context, size: Size(5, 5));

    AssetImage(icon)
        .resolve(config)
        .addListener(ImageStreamListener((ImageInfo image, bool sync) async {
      final ByteData? bytes = await image.image.toByteData(format: ImageByteFormat.png);
      final BitmapDescriptor bitmap = BitmapDescriptor.fromBytes(bytes!.buffer.asUint8List());
      bitmapIcon.complete(bitmap);
    })
    );

    return await bitmapIcon.future;
  }


		Marker(
        markerId: MarkerId(contact['name']),
        position: contact['position'],
        icon: await _getAssetIcon(context, 'assets/img/map/avatar-1.png').then((value) => value)
      );