getIterator({preventCancel = false} = {}) {
    return new ReadableStreamAsyncIterator(this.getReader(), preventCancel);
  }
Returns an AyncIterator over the ReadableStream.

If preventCancel is passed as an option, calling the return() method on the
iterator will terminate the iterator, but will not cancel the
ReadableStream.
https://streams.spec.whatwg.org/#rs-get-iterator
@param {{preventCancel: boolean}=} options
@return {!AsyncIterator<T>}
@override