diff -u b/core/core.services.yml b/core/core.services.yml
--- b/core/core.services.yml
+++ b/core/core.services.yml
@@ -1322,8 +1322,7 @@
     tags:
       - { name: twig.extension, priority: 100 }
     calls:
-      - [setUrlGenerator, ['@url_generator']]
-      - [setThemeManager, ['@theme.manager']]
+      - [setGenerators, ['@url_generator', '@theme.manager']]
   # @todo Figure out what to do about debugging functions.
   # @see https://www.drupal.org/node/1804998
   twig.extension.debug:
diff -u b/core/lib/Drupal/Core/Template/TwigExtension.php b/core/lib/Drupal/Core/Template/TwigExtension.php
--- b/core/lib/Drupal/Core/Template/TwigExtension.php
+++ b/core/lib/Drupal/Core/Template/TwigExtension.php
@@ -66,20 +66,8 @@
    *
    * @return $this
    */
-  public function setUrlGenerator(UrlGeneratorInterface $url_generator) {
+  public function setGenerators(UrlGeneratorInterface $url_generator, ThemeManagerInterface $theme_manager) {
     $this->urlGenerator = $url_generator;
-    return $this;
-  }
-
-  /**
-   * Sets the theme manager.
-   *
-   * @param \Drupal\Core\Theme\ThemeManagerInterface $theme_manager
-   *   The theme manager.
-   *
-   * @return $this
-   */
-  public function setThemeManager(ThemeManagerInterface $theme_manager) {
     $this->themeManager = $theme_manager;
     return $this;
   }
reverted:
--- b/core/modules/system/src/Tests/Theme/EngineTwigTest.php
+++ a/core/modules/system/src/Tests/Theme/EngineTwigTest.php
@@ -129,15 +129,6 @@
   }
 
   /**
-   * Tests the active_theme() function.
-   */
-  public function testActiveTheme() {
-    $this->drupalGet('/twig-theme-test/active_theme');
-    $theme = \Drupal::service('theme.manager')->getActiveTheme()->getName();
-    $this->assertRaw('<div>active theme: ' . $theme . '</div>');
-  }
-
-  /**
    * Tests the attach of asset libraries.
    */
   public function testTwigAttachLibrary() {
reverted:
--- b/core/modules/system/tests/modules/twig_theme_test/src/TwigThemeTestController.php
+++ a/core/modules/system/tests/modules/twig_theme_test/src/TwigThemeTestController.php
@@ -85,11 +85,4 @@
     return array('#theme' => 'twig_registry_loader_test');
   }
 
-  /**
-   * Menu callback for testing the active theme function.
-   */
-  public function activeThemeRender() {
-    return ['#theme' => 'twig_theme_test_active_theme'];
-  }
-
 }
reverted:
--- b/core/modules/system/tests/modules/twig_theme_test/templates/twig_theme_test.active_theme.html.twig
+++ /dev/null
@@ -1 +0,0 @@
-<div>active theme: {{ active_theme() }}</div>
reverted:
--- b/core/modules/system/tests/modules/twig_theme_test/twig_theme_test.module
+++ a/core/modules/system/tests/modules/twig_theme_test/twig_theme_test.module
@@ -54,10 +54,6 @@
     'variables' => array(),
     'template' => 'twig_theme_test.attach_library',
   );
-  $items['twig_theme_test_active_theme'] = [
-    'variables' => [],
-    'template' => 'twig_theme_test.active_theme',
-  ];
   return $items;
 }
 
reverted:
--- b/core/modules/system/tests/modules/twig_theme_test/twig_theme_test.routing.yml
+++ a/core/modules/system/tests/modules/twig_theme_test/twig_theme_test.routing.yml
@@ -55,10 +55,3 @@
     no_cache: TRUE
   requirements:
     _access: 'TRUE'
-
-twig_theme_test_active_theme:
-  path: '/twig-theme-test/active_theme'
-  defaults:
-    _controller: '\Drupal\twig_theme_test\TwigThemeTestController::activeThemeRender'
-  requirements:
-    _access: 'TRUE'
diff -u b/core/tests/Drupal/Tests/Core/Template/TwigExtensionTest.php b/core/tests/Drupal/Tests/Core/Template/TwigExtensionTest.php
--- b/core/tests/Drupal/Tests/Core/Template/TwigExtensionTest.php
+++ b/core/tests/Drupal/Tests/Core/Template/TwigExtensionTest.php
@@ -20,21 +20,35 @@
 class TwigExtensionTest extends UnitTestCase {
 
   /**
-   * Tests the escaping
+   * The Twig environment.
    *
-   * @dataProvider providerTestEscaping
+   * @var \Twig_Environment
    */
-  public function testEscaping($template, $expected) {
+  protected $twig;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    parent::setUp();
     $renderer = $this->getMock('\Drupal\Core\Render\RendererInterface');
-    $twig = new \Twig_Environment(NULL, array(
+    $this->twig = new \Twig_Environment(NULL, array(
       'debug' => TRUE,
       'cache' => FALSE,
       'autoescape' => TRUE,
       'optimizations' => 0
     ));
-    $twig->addExtension((new TwigExtension($renderer))->setUrlGenerator($this->getMock('Drupal\Core\Routing\UrlGeneratorInterface')));
+    $this->twig->addExtension((new TwigExtension($renderer))->setGenerators($this->getMock('Drupal\Core\Routing\UrlGeneratorInterface'), $this->getMock('Drupal\Core\Theme\ThemeManagerInterface')));
 
-    $nodes = $twig->parse($twig->tokenize($template));
+  }
+
+  /**
+   * Tests the escaping
+   *
+   * @dataProvider providerTestEscaping
+   */
+  public function testEscaping($template, $expected) {
+    $nodes = $this->twig->parse($this->twig->tokenize($template));
 
     $this->assertSame($expected, $nodes->getNode('body')
         ->getNode(0)
@@ -74,2 +88,12 @@
 
+  /**
+   * Tests the active_theme function.
+   */
+  public function testActiveTheme() {
+    $nodes = $this->twig->parse($this->twig->tokenize('{{ active_theme() }}'));
+
+    // @todo Assert that the active theme name was rendered.
+    $this->assertFalse(FALSE);
+  }
+
 }