checkPreloadOnDomElements_(interstitial) {
    if (this.preloadOnDomElements_.has(interstitial) ||
        (this.config_ && !this.config_.allowPreloadOnDomElements)) {
      return;
    }
    const createAndAddLink = (url) => {
      const link = /** @type {HTMLLinkElement} */(
        document.createElement('link'));
      link.rel = 'preload';
      link.href = url;
      link.as = 'image';
      document.head.appendChild(link);
      return link;
    };
    const links = [];
    if (interstitial.background) {
      const urlRegExp = /url\(('|")?([^'"()]+)('|")\)?/;
      const match = interstitial.background.match(urlRegExp);
      if (match) {
        links.push(createAndAddLink(match[2]));
      }
    }
    if (interstitial.mimeType.startsWith('image/')) {
      links.push(createAndAddLink(interstitial.uri));
    }
    this.preloadOnDomElements_.set(interstitial, links);
  }
@param {!shaka.extern.AdInterstitial} interstitial
@private