getIcuMessageFn = (message, values) => {
    const keyname = Object.keys(mergedStrings).find(key => mergedStrings[key] === message);
    if (!keyname) throw new Error(`Could not locate: ${message}`);

    // `message` can be a UIString defined within the provided `fileStrings`, or it could be
    // one of the common strings found in `i18n.UIStrings`.
    const filenameToLookup = keyname in fileStrings ?
      filename :
      path.relative(LH_ROOT, getModulePath(import.meta));
    const unixStyleFilename = filenameToLookup.replace(/\\/g, '/');
    const i18nId = `${unixStyleFilename} | ${keyname}`;

    return {
      i18nId,
      values,
      formattedDefault: formatMessage(message, values, DEFAULT_LOCALE),
    };
  }
Convert a message string and replacement values into an `LH.IcuMessage`.
@param {string} message
@param {Record<string, string | number>} [values]
@return {LH.IcuMessage}