code: 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;
  }
docs: Used for preventing the next write() call on TransformStreamSink until there is no longer backpressure.