docs: @VisibleForTesting
    static void pretendValidateResource(String resourceName, ContentKind kind) {
      int index = resourceName.lastIndexOf('.');
      Preconditions.checkArgument(
          index >= 0, "Currently, we only validate resources with explicit extensions.");
      String fileExtension = resourceName.substring(index + 1).toLowerCase();
  
      switch (kind) {
        case JS:
          Preconditions.checkArgument(fileExtension.equals("js"));
          break;
        case HTML:
          Preconditions.checkArgument(SAFE_HTML_FILE_EXTENSIONS.contains(fileExtension));
          break;
        case CSS:
          Preconditions.checkArgument(fileExtension.equals("css"));
          break;
        default:
          throw new IllegalArgumentException("Don't know how to validate resources of kind " + kind);
      }
    }