diff -u b/core/modules/system/tests/modules/theme_test/theme_test.module b/core/modules/system/tests/modules/theme_test/theme_test.module
--- b/core/modules/system/tests/modules/theme_test/theme_test.module
+++ b/core/modules/system/tests/modules/theme_test/theme_test.module
@@ -52,6 +52,11 @@
     'theme callback' => '_theme_custom_theme',
     'type' => MENU_CALLBACK,
   );
+  $items['theme-test/request-listener'] = array(
+    'page callback' => 'theme_test_request_listener_page_callback',
+    'access callback' => TRUE,
+    'type' => MENU_CALLBACK,
+  );
   $items['theme-test/template-test'] = array(
     'page callback' => 'theme_test_template_test_page_callback',
     'access callback' => TRUE,
@@ -74,6 +79,13 @@
 }
 
 /**
+ * Menu callback for testing themed output generated in a request listener.
+ */
+function theme_test_request_listener_page_callback() {
+  return $GLOBALS['theme_test_output'];
+}
+
+/**
  * Menu callback for testing template overridding based on filename.
  */
 function theme_test_template_test_page_callback() {
only in patch2:
unchanged:
--- /dev/null
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeEarlyInitializationTest.php
@@ -0,0 +1,43 @@
+<?php
+
+/**
+ * @file
+ * Definition of Drupal\system\Tests\Theme\ThemeEarlyInitializationTest.
+ */
+
+namespace Drupal\system\Tests\Theme;
+
+use Drupal\simpletest\WebTestBase;
+
+/**
+ * Functional test for initialization of the theme system early in the request.
+ */
+class ThemeEarlyInitializationTest extends WebTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  public static $modules = array('theme_test');
+
+  public static function getInfo() {
+    return array(
+      'name' => 'Early theme initialization',
+      'description' => 'Tests that the theme system can be correctly initialized early in the page request.',
+      'group' => 'Theme',
+    );
+  }
+
+  /**
+   * Test that the theme system can generate output in a request listener.
+   */
+  function testRequestListener() {
+    $this->drupalGet('theme-test/request-listener');
+    // Verify that themed output generated in the request listener appears.
+    $this->assertRaw('Themed output generated in a KernelEvents::REQUEST listener');
+    // Verify that the default theme's CSS still appears even though the theme
+    // system was initialized early.
+    $this->assertRaw('stark/css/layout.css');
+  }
+}
only in patch2:
unchanged:
--- /dev/null
+++ b/core/modules/system/tests/modules/theme_test/lib/Drupal/theme_test/EventSubscriber/ThemeTestSubscriber.php
@@ -0,0 +1,49 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\theme_test\EventSubscriber\ThemeTestSubscriber.
+ */
+
+namespace Drupal\theme_test\EventSubscriber;
+
+use Symfony\Component\HttpKernel\KernelEvents;
+use Symfony\Component\HttpKernel\Event\GetResponseEvent;
+use Symfony\Component\EventDispatcher\EventSubscriberInterface;
+
+/**
+ * Theme test subscriber for controller requests.
+ */
+class ThemeTestSubscriber implements EventSubscriberInterface {
+
+  /**
+   * Generates themed output early in a page request.
+   *
+   * @see Drupal\system\Tests\Theme\ThemeEarlyInitializationTest::testRequestListener()
+   */
+  public function onRequest(GetResponseEvent $event) {
+    $request = $event->getRequest();
+    $current_path = $request->attributes->get('system_path');
+    if ($current_path == 'theme-test/request-listener') {
+      // First, force the theme registry to be rebuilt on this page request.
+      // This allows us to test a full initialization of the theme system in
+      // the code below.
+      drupal_theme_rebuild();
+      // Next, initialize the theme system by storing themed text in a global
+      // variable. We will use this later in
+      // theme_test_request_listener_page_callback() to test that even when the
+      // theme system is initialized this early, it is still capable of
+      // returning output and theming the page as a whole.
+      $GLOBALS['theme_test_output'] = theme('more_link', array('url' => 'user', 'title' => 'Themed output generated in a KernelEvents::REQUEST listener'));
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  static function getSubscribedEvents() {
+    $events[KernelEvents::REQUEST][] = array('onRequest');
+    return $events;
+  }
+
+}
only in patch2:
unchanged:
--- /dev/null
+++ b/core/modules/system/tests/modules/theme_test/theme_test.services.yml
@@ -0,0 +1,5 @@
+services:
+  theme_test.subscriber:
+    class: Drupal\theme_test\EventSubscriber\ThemeTestSubscriber
+    tags:
+      - { name: event_subscriber }