code: func newUploadTracker(checker Checker, uploader Uploader, isol *isolated.Isolated) *UploadTracker {
  	isol.Files = make(map[string]isolated.File)
  	return &UploadTracker{
  		checker:       checker,
  		uploader:      uploader,
  		isol:          isol,
  		fileHashCache: make(map[string]hashResult),
  		lOS:           standardOS{},
  	}
  }
docs: // newUploadTracker constructs an UploadTracker.  It tracks uploaded files in isol.Files.