code: function Cluster(markerClusterer) {
    this.markerClusterer_ = markerClusterer;
    this.map_ = markerClusterer.getMap();
    this.gridSize_ = markerClusterer.getGridSize();
    this.minClusterSize_ = markerClusterer.getMinClusterSize();
    this.averageCenter_ = markerClusterer.isAverageCenter();
    this.center_ = null;
    this.markers_ = [];
    this.bounds_ = null;
    this.clusterIcon_ = new ClusterIcon(this, markerClusterer.getStyles(),
        markerClusterer.getGridSize());
  }
docs: A cluster that contains markers.
  
  @param {MarkerClusterer} markerClusterer The markerclusterer that this
  cluster is associated with.
  @constructor
  @ignore