Flutter: How to add marker to Google Maps with new Marker API?

Yes, The google maps API has changed and the Marker API is widget based and not based in controller anymore.

By CHANGELOG.md

“Breaking change. Changed the Marker API to be widget based, it was controller based. Also changed the example app to account for the same.”

I copy some pieces of code from github app example that I think is important to you

Map<MarkerId, Marker> markers = <MarkerId, Marker>{}; // CLASS MEMBER, MAP OF MARKS

void _add() {
    var markerIdVal = MyWayToGenerateId();
    final MarkerId markerId = MarkerId(markerIdVal);

    // creating a new MARKER
    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);
      },
    );

    setState(() {
      // adding a new marker to map
      markers[markerId] = marker;
    });
}

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), // YOUR MARKS IN MAP
)

I advise you take a look in example app here. There is updated to new API.