function makeDefaultFile(fileName) {
  const localeForFileName = getLocaleForFileName(DEFAULT_LOCALE);
  const path = `${DEST_DIR}/${fileName}__${localeForFileName}.js`;
  if (fse.existsSync(path)) {
    return fse.copy(path, `${DEST_DIR}/${fileName}.js`);
  }
}
Creates the default FirebaseUI binaries for basic usage without
localization. For example, it copies firebaseui__en.js to firebaseui.js.
@param {string} fileName
@return {!Promise} A promise that resolves on completion.