code: function bundlePreprocessor(config) {
  
      var debug = config.browserify && config.browserify.debug;
  
      function updateSourceMap(file, content) {
        var map;
  
        if (debug) {
  
          map = extractSourceMap(content);
  
          file.sourceMap = map && map.sourcemap;
        }
      }
  
      return function(content, file, done) {
  
        if (b._builtOnce) {
          updateSourceMap(file, content);
          return done(content);
        }
  
        log.debug('building bundle');
  
        // wait for the initial bundle to be created
        b.deferredBundle(function(err, content) {
  
          b._builtOnce = config.autoWatch;
  
          if (err) {
            return done(BUNDLE_ERROR_TPL);
          }
  
          content = content.toString('utf-8');
          updateSourceMap(file, content);
  
          log.info('bundle built');
  
          done(content);
        });
      };
    }
docs: A special preprocessor that builds the main browserify bundle once and
  passes the bundle contents through on all later preprocessing request.