storeReplacement_(replacement) {
    this.replacementsCount_++;
    const marker =
        `{${MARKER}` + this.replacementsCount_ + '_' + getRandomString() + '}';
    this.replacements_[htmlEscape(marker)] = replacement;
    return marker;
  }
Stores a replacement and returns its marker.
@param {!SafeHtmlFormatter.Replacement} replacement
@return {string} Marker.
@private