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