code: func appEngineTokenSource(ctx context.Context, scope ...string) oauth2.TokenSource {
  	scopes := append([]string{}, scope...)
  	sort.Strings(scopes)
  	return &gaeTokenSource{
  		ctx:    ctx,
  		scopes: scopes,
  		key:    strings.Join(scopes, " "),
  	}
  }
docs: // See comment on AppEngineTokenSource in appengine.go.