ReadableStream API exposed for controllers.
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;
}