code: function () {
        var tail = this._tail
          , head = this._head;
        for (var i = head - 1; i >= tail; i--) {
          var headEntry = this._byOrder[i];
          if (headEntry) {
            this._head = i + 1;
            return headEntry;
          }
        }
      }
docs: update head index
  @return {Entry|undefined}
  @api private