docs: function writeVendorFileThenBundle () {
      const vendorDestPath = path.join(opts.rootDir, outputOpts.dir, outputOpts.vendor)
      const outStream = fs.createWriteStream(vendorDestPath)
  
      var b = getBrowserifyInstance({
        inc: opts.inc,
        cacheFile: opts.cacheFile
      })
  
      // add vendor files to the bundle
      b.require(opts.vendor)
  
      const startTime = Date.now()
      const bStream = b.bundle()
  
      multistream(vendorFiles.map(createReadStream).concat(bStream))
        .pipe(outStream)
        .on('error', (err) => console.error(err))
        .on('close', function () {
          const endTime = Date.now()
          const delta = endTime - startTime
  
          console.error(
            '[vendor] written to %s (%s seconds)',
            vendorDestPath, (delta / 1000).toFixed(2)
          )
  
          bundle()
        })
    }