next() {
    if (!this.asyncIteratorReader.ownerReadableStream) {
      return Promise.reject(
          new TypeError('There is no more data left in the ReadableStream'));
    }
    return this.asyncIteratorReader.read().then(({value, done}) => {
      if (done) {
        this.asyncIteratorReader.release();
      }
      return {value, done};
    });
  }
Gets the next value from the ReadableStream.
https://streams.spec.whatwg.org/#rs-asynciterator-prototype-next
@override