code: function updateOneLayer(item, currZIndex) {
    if (defined(item.imageryLayer) && defined(item.imageryLayer.setZIndex)) {
      if (item.supportsReordering) {
        item.imageryLayer.setZIndex(currZIndex.reorderable++);
      } else {
        item.imageryLayer.setZIndex(currZIndex.fixed++);
      }
    }
  }
docs: this private function is called by updateLayerOrder