getNextSafeSlug(originalSlug, isDryRun) {
    let slug = originalSlug;
    let occurenceAccumulator = 0;
    if (this.seen.hasOwnProperty(slug)) {
      occurenceAccumulator = this.seen[originalSlug];
      do {
        occurenceAccumulator++;
        slug = originalSlug + '-' + occurenceAccumulator;
      } while (this.seen.hasOwnProperty(slug));
    }
    if (!isDryRun) {
      this.seen[originalSlug] = occurenceAccumulator;
      this.seen[slug] = 0;
    }
    return slug;
  }
Finds the next safe (unique) slug to use