code: function ReadableStreamAddReadIntoRequest(stream) {
    assert(IsReadableStreamBYOBReader(stream._reader) === true);
    assert(stream._state === 'readable' || stream._state === 'closed');
  
    const promise = new Promise((resolve, reject) => {
      const readIntoRequest = {
        _resolve: resolve,
        _reject: reject
      };
  
      stream._reader._readIntoRequests.push(readIntoRequest);
    });
  
    return promise;
  }
docs: ReadableStream API exposed for controllers.