Move DrupalApp back to being a kernel implementation, since, really, that's what it is.

core/lib/Drupal/Core/DrupalApp.php

@@ -29,7 +29,7 @@
 /**
  * The DrupalApp class is the core of Drupal itself.
  */
-class DrupalApp {
+class DrupalApp implements HttpKernelInterface {
 
   /**
    *
@@ -38,7 +38,7 @@ class DrupalApp {
    * @return Response
    *   The response object to return to the requesting user agent.
    */
-  function execute(Request $request) {
+  function handle(Request $request, $type = self::MASTER_REQUEST, $catch = true) {
     try {
 
       $dispatcher = $this->getDispatcher();

index.php

@@ -27,4 +27,4 @@
 $request = Request::createFromGlobals();
 
 $kernel = new DrupalApp();
-$kernel->execute($request)->send();
+$kernel->handle($request)->send();