static join(separator, parts) {
    const separatorHtml = SafeHtml.htmlEscape(separator);
    const content = [];

    /**
     * @param {!SafeHtml.TextOrHtml_|
     *     !Array<!SafeHtml.TextOrHtml_>} argument
     */
    const addArgument = (argument) => {
      if (Array.isArray(argument)) {
        argument.forEach(addArgument);
      } else {
        const html = SafeHtml.htmlEscape(argument);
        content.push(SafeHtml.unwrap(html));
      }
    };

    parts.forEach(addArgument);
    return SafeHtml.createSafeHtmlSecurityPrivateDoNotAccessOrElse(
        content.join(SafeHtml.unwrap(separatorHtml)));
  }
Creates a new SafeHtml object by joining the parts with separator.
@param {!SafeHtml.TextOrHtml_} separator
@param {!Array<!SafeHtml.TextOrHtml_|
    !Array<!SafeHtml.TextOrHtml_>>} parts Parts to join. If a part
    contains an array then each member of this array is also joined with
the separator.
@return {!SafeHtml}
@deprecated Use `safevalues.joinHtmls` instead.