code: 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`);
    }
  }
docs: 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.