code: func appEngineTokenSource(ctx context.Context, scope ...string) oauth2.TokenSource {
  	logOnce.Do(func() {
  		log.Print("google: AppEngineTokenSource is deprecated on App Engine standard second generation runtimes (>= Go 1.11) and App Engine flexible. Please use DefaultTokenSource or ComputeTokenSource.")
  	})
  	return ComputeTokenSource("")
  }
docs: // See comment on AppEngineTokenSource in appengine.go.