dart-marker-clusterer-plus example

googleMaps.GMap _map = new googleMaps.GMap(_mapDiv);
MarkerClusterer _clusterer =  = new MarkerClusterer(_map, [], new MarkerClustererOptions()
        ..maxZoom = 17
        ..minimumClusterSize = 4
        ..averageCenter = true
        ..gridSize = 100
        ..clusterClass = "cluster"
        ..styles = [new ClusterIconStyle()
            ..url = "images/cluster.svg"
            ..height = 32
            ..width = 32]);

googleMaps.Marker marker = new googleMaps.Marker()..onClick.listen((googleMaps.MouseEvent e) => onClick(mapMarker));
_clusterer.addMarker(marker, true);
_clusterer.repaint();