How to add multiple markers inside google maps of flutter

I am trying to add more than one marker in my google maps screen.. but I didn't get any idea how to do that.. I have applied some examples in some articles.. but I get an error...here is part of my code..

GoogleMap(
              initialCameraPosition: const CameraPosition(
                target: LatLng(...,...),
                zoom: 18.0,
              ),
              markers: Set.of([marker]), // here I want to add multiple markers
              onMapCreated: _onMapCreated,
)

is there a simple way to add multiple markers?

and here is the code for marker

void loc(LocationData a) {
    LatLng latlng = LatLng(a.latitude, a.longitude);
    this.setState(() {
      marker = Marker(
        markerId: MarkerId("mine"),
        position: latlng,
        icon: BitmapDescriptor.defaultMarker,
        draggable: false,
        zIndex: 1,
      );
    });
  }

google_maps_flutter has provided example for adding multiple markers: place marker, you should check this out:

  void _add() {
    final int markerCount = markers.length;

    if (markerCount == 12) {
      return;
    }

    final String markerIdVal = 'marker_id_$_markerIdCounter';
    _markerIdCounter++;
    final MarkerId markerId = MarkerId(markerIdVal);

    final Marker marker = Marker(
      markerId: markerId,
      position: LatLng(
        center.latitude + sin(_markerIdCounter * pi / 6.0) / 20.0,
        center.longitude + cos(_markerIdCounter * pi / 6.0) / 20.0,
      ),
      infoWindow: InfoWindow(title: markerIdVal, snippet: '*'),
      onTap: () {
        _onMarkerTapped(markerId);
      },
      onDragEnd: (LatLng position) {
        _onMarkerDragEnd(markerId, position);
      },
    );

    setState(() {
      markers[markerId] = marker;
    });
  }

UPDATED:

GoogleMap(
              onMapCreated: _onMapCreated,
              initialCameraPosition: const CameraPosition(
                target: LatLng(-33.852, 151.211),
                zoom: 11.0,
              ),
              // TODO(iskakaushik): Remove this when collection literals makes it to stable.
              // https://github.com/flutter/flutter/issues/28312
              // ignore: prefer_collection_literals
              markers: Set<Marker>.of(markers.values),
            ),

void loc(LocationData a) {
    LatLng latlng = LatLng(a.latitude, a.longitude);
     final Marker marker = Marker(
        markerId: MarkerId("mine"),
        position: latlng,
        icon: BitmapDescriptor.defaultMarker,
        draggable: false,
        zIndex: 1,
      );
    setState(() {
      markers[markerId] = marker;
    });
  }