code: func AppEngineTokenSource(ctx context.Context, scope ...string) oauth2.TokenSource {
  	if appengineTokenFunc == nil {
  		panic("google: AppEngineTokenSource can only be used on App Engine.")
  	}
  	scopes := append([]string{}, scope...)
  	sort.Strings(scopes)
  	return &appEngineTokenSource{
  		ctx:    ctx,
  		scopes: scopes,
  		key:    strings.Join(scopes, " "),
  	}
  }
docs: // AppEngineTokenSource returns a token source that fetches tokens
  // issued to the current App Engine application's service account.
  // If you are implementing a 3-legged OAuth 2.0 flow on App Engine
  // that involves user accounts, see oauth2.Config instead.
  //
  // The provided context must have come from appengine.NewContext.