function isIcuMessage(icuMessageOrNot) {
  if (!isObjectOfUnknownValues(icuMessageOrNot)) {
    return false;
  }

  const {i18nId, values, formattedDefault} = icuMessageOrNot;
  if (typeof i18nId !== 'string') {
    return false;
  }

  // formattedDefault is required.
  if (typeof formattedDefault !== 'string') {
    return false;
  }

  // Values is optional.
  if (values !== undefined) {
    if (!isObjectOfUnknownValues(values)) {
      return false;
    }
    for (const value of Object.values(values)) {
      if (typeof value !== 'string' && typeof value !== 'number') {
        return false;
      }
    }
  }

  // Finally return true if i18nId seems correct.
  return MESSAGE_I18N_ID_REGEX.test(i18nId);
}
Returns whether `icuMessageOrNot`` is an `LH.IcuMessage` instance.
@param {unknown} icuMessageOrNot
@return {icuMessageOrNot is LH.IcuMessage}