gns must be escaped as $$.
   * @param {string} label A localized string to substitute through.
   *     This is not an resource ID.
   * @param {...(string|number)} var_args The extra values to include in the
   *     formatted output.
   * @return {!Array<!{value: string, arg: (null|string)}>} The formatted
   *     string pieces.
   */
        getSubstitutedStringPieces(label, var_args) {
            const varArgs = arguments;
            // Split the string by separately matching all occurrences of $1-9