Issue #2612876 by leolandotan, NickDickinsonWilde, victoria-marina,...

  }

  /**

   * Build aggregate CSS file.

   * Processes CSS file and adds base URLs to any relative resource paths.

   *

   * @param array $css_asset

   *   A CSS asset. The array should contain the `data` key where the value

   *   should be the path to the CSS file relative to the Drupal root. This is

   *   an example of the `data` key's value,

   *   "core/assets/vendor/normalize-css/normalize.css".

   *

   * @return string

   *   The asset's cleaned/optimized contents.

   */

  protected function processFile($css_asset) {

    $contents = $this->loadFile($css_asset['data'], TRUE);