code: public TranspileResult embedSourcemapUrl(String url) {
      // If there's no sourcemap, don't reference it.
      if (sourceMap.isEmpty()) {
        return this;
      }
      String embedded = transpiled + "\n//# sourceMappingURL=" + url + "\n";
      return new TranspileResult(path, original, embedded, sourceMap);
    }
docs: TODO(sdh): might be a bit nicer for this to be a suffix rather than a whole path?