class FileLocator

Kernel.php in drupal/core/vendor/symfony/http-kernel/Symfony/Component/HttpKernel/Kernel.php

class FileLocator extends BaseFileLocator {
  private $kernel;
  private $path;

  /**
   * Constructor.
   *
   * @param KernelInterface $kernel A KernelInterface instance
   * @param string          $path   The path the global resource directory
   * @param string|array    $paths  A path or an array of paths where to look for resources
   */
  public function __construct(KernelInterface $kernel, $path = null, array $paths = array()) {
    $this->kernel = $kernel;
    $this->path = $path;
    $paths[] = $path;
    parent::__construct($paths);
  }

  /**
   * {@inheritdoc}
   */
  public function locate($file, $currentPath = null, $first = true) {
    if ('@' === $file[0]) {
      return $this->kernel
        ->locateResource($file, $this->path, $first);
    }
    return parent::locate($file, $currentPath, $first);
  }

}