class HttpAsset

AssetFactory.php in drupal/core/vendor/kriswallsmith/assetic/src/Assetic/Factory/AssetFactory.php

CssImportFilter.php in drupal/core/vendor/kriswallsmith/assetic/src/Assetic/Filter/CssImportFilter.php

class HttpAsset extends BaseAsset {
  private $sourceUrl;
  private $ignoreErrors;

  /**
   * Constructor.
   *
   * @param string  $sourceUrl    The source URL
   * @param array   $filters      An array of filters
   * @param Boolean $ignoreErrors
   * @param array   $vars
   *
   * @throws \InvalidArgumentException If the first argument is not an URL
   */
  public function __construct($sourceUrl, $filters = array(), $ignoreErrors = false, array $vars = array()) {
    if (0 === strpos($sourceUrl, '//')) {
      $sourceUrl = 'http:' . $sourceUrl;
    }
    elseif (false === strpos($sourceUrl, '://')) {
      throw new \InvalidArgumentException(sprintf('"%s" is not a valid URL.', $sourceUrl));
    }
    $this->sourceUrl = $sourceUrl;
    $this->ignoreErrors = $ignoreErrors;
    list($scheme, $url) = explode('://', $sourceUrl, 2);
    list($host, $path) = explode('/', $url, 2);
    parent::__construct($filters, $scheme . '://' . $host, $path, $vars);
  }
  public function load(FilterInterface $additionalFilter = null) {
    $content = @file_get_contents(VarUtils::resolve($this->sourceUrl, $this
      ->getVars(), $this
      ->getValues()));
    if (false === $content && !$this->ignoreErrors) {
      throw new \RuntimeException(sprintf('Unable to load asset from URL "%s"', $this->sourceUrl));
    }
    $this
      ->doLoad($content, $additionalFilter);
  }
  public function getLastModified() {
    if (false !== @file_get_contents($this->sourceUrl, false, stream_context_create(array(
      'http' => array(
        'method' => 'HEAD',
      ),
    )))) {
      foreach ($http_response_header as $header) {
        if (0 === stripos($header, 'Last-Modified: ')) {
          list(, $mtime) = explode(':', $header, 2);
          return strtotime(trim($mtime));
        }
      }
    }
  }

}