code: func AppEngineTokenSource(ctx context.Context, scope ...string) oauth2.TokenSource {
  	return appEngineTokenSource(ctx, scope...)
  }
docs: // AppEngineTokenSource returns a token source that fetches tokens from either
  // the current application's service account or from the metadata server,
  // depending on the App Engine environment. See below for environment-specific
  // details. If you are implementing a 3-legged OAuth 2.0 flow on App Engine that
  // involves user accounts, see oauth2.Config instead.
  //
  // First generation App Engine runtimes (<= Go 1.9):
  // AppEngineTokenSource returns a token source that fetches tokens issued to the
  // current App Engine application's service account. The provided context must have
  // come from appengine.NewContext.
  //
  // Second generation App Engine runtimes (>= Go 1.11) and App Engine flexible:
  // AppEngineTokenSource is DEPRECATED on second generation runtimes and on the
  // flexible environment. It delegates to ComputeTokenSource, and the provided
  // context and scopes are not used. Please use DefaultTokenSource (or ComputeTokenSource,
  // which DefaultTokenSource will use in this case) instead.