code: function Cluster(mc) {
    this.markerClusterer_ = mc;
    this.map_ = mc.getMap();
    this.gridSize_ = mc.getGridSize();
    this.minClusterSize_ = mc.getMinimumClusterSize();
    this.averageCenter_ = mc.getAverageCenter();
    this.markers_ = [];
    this.center_ = null;
    this.bounds_ = null;
    this.clusterIcon_ = new ClusterIcon(this, mc.getStyles());
  }
docs: Creates a single cluster that manages a group of proximate markers.
  Used internally, do not call this constructor directly.
  @constructor
  @param {MarkerClusterer} mc The <code>MarkerClusterer</code> object with which this
  cluster is associated.