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
@@ -20,35 +20,21 @@
 class TwigExtensionTest extends UnitTestCase {
 
   /**
-   * The Twig environment.
+   * Tests the escaping
    *
-   * @var \Twig_Environment
-   */
-  protected $twig;
-
-  /**
-   * {@inheritdoc}
+   * @dataProvider providerTestEscaping
    */
-  protected function setUp() {
-    parent::setUp();
+  public function testEscaping($template, $expected) {
     $renderer = $this->getMock('\Drupal\Core\Render\RendererInterface');
-    $this->twig = new \Twig_Environment(NULL, array(
+    $twig = new \Twig_Environment(NULL, array(
       'debug' => TRUE,
       'cache' => FALSE,
       'autoescape' => TRUE,
       'optimizations' => 0
     ));
-    $this->twig->addExtension((new TwigExtension($renderer))->setGenerators($this->getMock('Drupal\Core\Routing\UrlGeneratorInterface'), $this->getMock('Drupal\Core\Theme\ThemeManagerInterface')));
-
-  }
+    $twig->addExtension((new TwigExtension($renderer))->setGenerators($this->getMock('Drupal\Core\Routing\UrlGeneratorInterface'), $this->getMock('Drupal\Core\Theme\ThemeManagerInterface')));
 
-  /**
-   * Tests the escaping
-   *
-   * @dataProvider providerTestEscaping
-   */
-  public function testEscaping($template, $expected) {
-    $nodes = $this->twig->parse($this->twig->tokenize($template));
+    $nodes = $twig->parse($twig->tokenize($template));
 
     $this->assertSame($expected, $nodes->getNode('body')
         ->getNode(0)
@@ -90,10 +76,28 @@
    * 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);
+    $renderer = $this->getMock('\Drupal\Core\Render\RendererInterface');
+    $extension = new TwigExtension($renderer);
+    $url_generator = $this->getMock('\Drupal\Core\Routing\UrlGeneratorInterface');
+    $theme_manager = $this->getMock('\Drupal\Core\Theme\ThemeManagerInterface');
+    $active_theme = $this->getMockBuilder('\Drupal\Core\Theme\ActiveTheme')
+      ->disableOriginalConstructor()
+      ->getMock();
+    $active_theme
+      ->expects($this->once())
+      ->method('getName')
+      ->willReturn('test_theme');
+    $theme_manager
+      ->expects($this->once())
+      ->method('getActiveTheme')
+      ->willReturn($active_theme);
+    $extension->setGenerators($url_generator, $theme_manager);
+
+    $loader = new \Twig_Loader_String();
+    $twig = new \Twig_Environment($loader);
+    $twig->addExtension($extension);
+    $result = $twig->render('{{ active_theme() }}');
+    $this->assertEquals('test_theme', $result);
   }
 
 }