google-map-markercluster-icon-behavior.html

<script>
  /**
   * Use `ClusterIconBehavior` to implement elements that should display a custom cluster icon.
   * Users should implement the `updateMarkers` function if they need to customize based on the markers that are contained
   * in the cluster.
   * @polymer
   * @mixinFunction
  */
  ClusterIconBehavior = function(superClass) {
    return class extends superClass {
      static get properties() {
        return {
          /**
           * The markers that are contained in the cluster.
           */
          markers: {
            type: Array,
            observer: 'updateMarkers'
          }
        }
      }
      /**
       * Observer function that is called when the `markers` Array is changed.
       */
      updateMarkers(markers) {
		  	//abstract
      }
    }
  }
</script>