cancel(reason) {
    if (!this.ownerReadableStream) {
      return Promise.reject(new TypeError(
          'This readable stream reader has been released and cannot be used ' +
          'to cancel its previous owner stream'));
    }
    return /** @type {!ReadableStream} */ (this.ownerReadableStream)
        .cancelInternal(reason);
  }
Cancels the ReadableStream with an optional reason.
https://streams.spec.whatwg.org/#default-reader-cancel
@param {*} reason
@return {!Promise<undefined>}
@override