code: function Flow(flowControlId) {
    Duplex.call(this, { objectMode: true });
  
    this._window = this._initialWindow = INITIAL_WINDOW_SIZE;
    this._flowControlId = flowControlId;
    this._queue = [];
    this._ended = false;
    this._received = 0;
    this._blocked = false;
  }
docs: `flowControlId` is needed if only specific WINDOW_UPDATEs should be watched.