Add a reference to the kernel in ExceptionController.

core/lib/Drupal/Core/DrupalKernel.php

@@ -67,6 +67,6 @@ public function __construct(EventDispatcherInterface $dispatcher, ControllerReso
       // Either way, treat it as a server-level error and return an HTTP 500.
       // By default, this will be an HTML-type response because that's a decent
       // best guess if we don't know otherwise.
-      $this->dispatcher->addSubscriber(new ExceptionListener(array(new ExceptionController($negotiation), 'execute')));
+      $this->dispatcher->addSubscriber(new ExceptionListener(array(new ExceptionController($this, $negotiation), 'execute')));
     }
 }

core/lib/Drupal/Core/ExceptionController.php

@@ -7,6 +7,8 @@
 use Symfony\Component\HttpFoundation\Response;
 
 use Symfony\Component\HttpKernel\HttpKernelInterface;
+use Symfony\Component\EventDispatcher\EventDispatcher;
+use Symfony\Component\HttpKernel\Controller\ControllerResolver;
 use Symfony\Component\HttpKernel\Exception\FlattenException;
 
 use Exception;
@@ -17,9 +19,11 @@
  */
 class ExceptionController {
 
+  protected $kernel;
   protected $negotiation;
 
-  public function __construct(ContentNegotiation $negotiation) {
+  public function __construct(HttpKernelInterface $kernel, ContentNegotiation $negotiation) {
+    $this->kernel = $kernel;
     $this->negotiation = $negotiation;
   }
 
@@ -73,10 +77,9 @@ public function on404Html(FlattenException $exception, Request $request) {
     if ($path && $path != $system_path) {
       // @TODO: Um, how do I specify an override URL again? Totally not clear.
       // Do that and sub-call the kernel rather than using meah().
-      $request = Request::create($path);
+      $subrequest = Request::create('/' . $path, 'get', array(), $request->cookies->all(), array(), $request->server->all());
 
-      $kernel = new DrupalKernel();
-      $response = $kernel->handle($request, HttpKernelInterface::SUB_REQUEST);
+      $response = $this->kernel->handle($subrequest, HttpKernelInterface::SUB_REQUEST);
       $response->setStatusCode(404, 'Not Found');
     }
     else {