code: function serveSassFiles(magnet, outputDir) {
    if (!isServingSassFiles) {
      magnet.getServer()
        .getEngine()
        .use('/css', express.static(outputDir));
    }
    isServingSassFiles = true;
  }
docs: Serves built sass files using magnet server instance
  @param {!Magnet} magnet
  @param {!string} outputDir