set(requestId) {
    if (this._pendingRequests[requestId]) {
      return;
    }
    /** @type {(value?: any) => void } */
    let resolve;
    const promise = new Promise(_resolve => {
      resolve = _resolve;
    });
    // @ts-ignore
    this._pendingRequests[requestId] = { promise, resolve };
  }
Creates a promise for a pending request with given key
@param {string} requestId