replacements() {
    if (this.settings.replacements === "none") {
      return new Promise(function (resolve) {
        resolve(this.urls);
      }.bind(this));
    }

    var replacements = this.urls.map(url => {
      var absolute = this.settings.resolver(url);
      return this.createUrl(absolute).catch(err => {
        console.error(err);
        return null;
      });
    });
    return Promise.all(replacements).then(replacementUrls => {
      this.replacementUrls = replacementUrls.filter(url => {
        return typeof url === "string";
      });
      return replacementUrls;
    });
  }
Create blob urls for all the assets
@return {Promise}         returns replacement urls