simple-lru.js
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;
        }
      }
    }
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;
        }
      }
    }
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 ; } } }
update tail index
@return {Entry|undefined}
@api private
update tail index