Used for preventing the next write() call on TransformStreamSink until there is no longer backpressure.
function TransformStreamReadableReadyPromise(transformStream) {
  assert(transformStream._backpressureChangePromise !== undefined,
         '_backpressureChangePromise should have been initialized');

  if (transformStream._backpressure === false) {
    return Promise.resolve();
  }

  assert(transformStream._backpressure === true, '_backpressure should have been initialized');

  return transformStream._backpressureChangePromise;
}