code: function() {
          var index = this.layer.getZIndex() - 1;
          if (index >= this.map.Z_INDEX_BASE['Feature']) {
              this.layer.setZIndex(index);
          } else {
              this.map.setLayerZIndex(this.layer,
                  this.map.getLayerIndex(this.layer));
          }
      }
docs: Method: moveLayerBack
  Moves the layer back to the position determined by the map's layers
  array.