code: function (o1, o2) {
  
              var zIndex1 = (o1.cfg) ? o1.cfg.getProperty("zIndex") : null, // Sort invalid (destroyed)
                  zIndex2 = (o2.cfg) ? o2.cfg.getProperty("zIndex") : null; // objects at bottom.
  
              if (zIndex1 === null && zIndex2 === null) {
                  return 0;
              } else if (zIndex1 === null){
                  return 1;
              } else if (zIndex2 === null) {
                  return -1;
              } else if (zIndex1 > zIndex2) {
                  return -1;
              } else if (zIndex1 < zIndex2) {
                  return 1;
              } else {
                  return 0;
              }
          }
docs: Used for sorting the manager's Overlays by z-index.
  @method compareZIndexDesc
  @private
  @return {Number} 0, 1, or -1, depending on where the Overlay should
  fall in the stacking order.