docs: function start(userConfig) {
      var config = _.assign(karmaDefaultConfig(), userConfig);
      config = DefaultsDeep(config, KarmaConfig());
      if (!_.isArray(config.files) || _.isEmpty(config.files)) {
          throw new Error('no files provided');
          return null;
      }
  
      var promise = new Promise(function (resolve, reject) {
          server.start(config, function () {
              resolve(this);
          });
      });
      return promise;
  }