Methods on the transform stream controller object
function TransformStreamCloseReadable(transformStream) {
  // console.log('TransformStreamCloseReadable()');

  if (transformStream._errored === true) {
    throw new TypeError('TransformStream is already errored');
  }

  if (transformStream._readableClosed === true) {
    throw new TypeError('Readable side is already closed');
  }

  TransformStreamCloseReadableInternal(transformStream);
}