async function readdirRecursive(path) {
  const filesAndDirectories =
      (await fs.readdir(path))
          .map(fileOrDirectory => `${path}/${fileOrDirectory}`)
          .sort();
  const files = [];
  for (const fileOrDirectory of filesAndDirectories) {
    const fileStat = await fs.stat(fileOrDirectory);
    if (fileStat.isDirectory()) {
      files.push(...await readdirRecursive(fileOrDirectory));
    } else {
      files.push(fileOrDirectory);
    }
  }
  return files;
}
Calls fs.readdir recursively on the given path and subdirectories.
Returns only the list of files.
@param {string} path The path to read.
@return {!Array<string>} A list of files.