endTag(tagName) {
    SafeHtml.verifyTagName(tagName);
    return this.storeReplacement_({endTag: tagName});
  }
Saves an end tag and returns its marker.
@param {string} tagName
@return {string} Marker.
@throws {!Error} If invalid tag name, attribute name, or attribute value is
    provided. This function accepts the same tags as {@link
    SafeHtml.create}.