code: function (layers) {
  		var id, parent;
  
  		// Assumes layers is an Array or an Object whose prototype is non-enumerable.
  		for (id in layers) {
  			// Flag parent clusters' icon as "dirty", all the way up.
  			// Dumb process that flags multiple times upper parents, but still
  			// much more efficient than trying to be smart and make short lists,
  			// at least in the case of a hierarchy following a power law:
  			// http://jsperf.com/flag-nodes-in-power-hierarchy/2
  			parent = layers[id].__parent;
  			while (parent) {
  				parent._iconNeedsUpdate = true;
  				parent = parent.__parent;
  			}
  		}
  	}
docs: Simply flags all parent clusters of the given markers as having a "dirty" icon.
  @param layers Array(L.Marker)|Map(L.Marker) list of markers.
  @private