diff --git a/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php
index 20ccc2b..73a4eb6 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php
@@ -174,7 +174,7 @@ function testThemeGetSetting() {
    * Ensures the theme registry is rebuilt when modules are disabled/enabled.
    */
   function testRegistryRebuild() {
-    $this->assertIdentical(theme('theme_test_foo', array('foo' => 'a')), 'a', 'The theme registry contains theme_test_foo.');
+    $this->assertIdentical(trim(theme('theme_test_foo', array('foo' => 'a'))), 'a', 'The theme registry contains theme_test_foo.');
 
     module_disable(array('theme_test'), FALSE);
     // After enabling/disabling a module during a test, we need to rebuild the
@@ -190,7 +190,7 @@ function testRegistryRebuild() {
     // throws an exception.
     $this->rebuildContainer();
     $this->container->get('module_handler')->loadAll();
-    $this->assertIdentical(theme('theme_test_foo', array('foo' => 'c')), 'c', 'The theme registry contains theme_test_foo again after re-enabling the module.');
+    $this->assertIdentical(trim(theme('theme_test_foo', array('foo' => 'c'))), 'c', 'The theme registry contains theme_test_foo again after re-enabling the module.');
   }
 
   /**
diff --git a/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTestTwig.php b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTestTwig.php
index c62f7c3..87b9fcd 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTestTwig.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTestTwig.php
@@ -63,8 +63,8 @@ function testFindThemeTemplates() {
     $this->assertEqual($cache['node']['template_file'], 'core/modules/node/templates/node.html.twig', 'Node is using node.html.twig as template file');
     $this->assertEqual($cache['node']['engine'], 'twig', 'Node is using twig engine');
 
-    $this->assertEqual($cache['theme_test_template_test']['template_file'], 'core/modules/system/tests/modules/theme_test/templates/theme_test.template_test.tpl.php', 'theme_test is using theme_test.template_test.tpl.php as template file');
-    $this->assertEqual($cache['theme_test_template_test']['engine'], 'phptemplate', 'theme_test is using phptemplate as engine.');
+    $this->assertEqual($cache['theme_test_template_test']['template_file'], 'core/modules/system/tests/modules/theme_test/templates/theme_test.template_test.html.twig', 'theme_test is using theme_test.template_test.html.twig as template file');
+    $this->assertEqual($cache['theme_test_template_test']['engine'], 'twig', 'theme_test is using twig as engine.');
 
     $templates = drupal_find_theme_templates($cache, '.html.twig', drupal_get_path('theme', 'test_theme_twig'));
     $this->assertEqual($templates['node__1']['template'], 'node--1', 'Template node--1.html.twig was found in test_theme_twig.');
diff --git a/core/modules/system/tests/modules/theme_test/templates/theme-test-foo.html.twig b/core/modules/system/tests/modules/theme_test/templates/theme-test-foo.html.twig
new file mode 100644
index 0000000..c4e8f72
--- /dev/null
+++ b/core/modules/system/tests/modules/theme_test/templates/theme-test-foo.html.twig
@@ -0,0 +1,14 @@
+{#
+/**
+ * @file
+ * Default theme implementation for the 'theme_test_foo' theme hook.
+ *
+ * Available variables:
+ * - foo: Dummy text to display.
+ *
+ * @see template_preprocess()
+ *
+ * @ingroup themeable
+ */
+#}
+{{ foo }}
diff --git a/core/modules/system/tests/modules/theme_test/templates/theme-test.html.twig b/core/modules/system/tests/modules/theme_test/templates/theme-test.html.twig
new file mode 100644
index 0000000..c5c9b8d
--- /dev/null
+++ b/core/modules/system/tests/modules/theme_test/templates/theme-test.html.twig
@@ -0,0 +1,15 @@
+{#
+/**
+ * @file
+ * Default theme implementation for the 'theme_test' theme hook.
+ *
+ * Available variables:
+ * - foo: Dummy text to display.
+ *
+ * @see template_preprocess()
+ * @see template_preprocess_theme_test()
+ *
+ * @ingroup themeable
+ */
+#}
+Theme hook implementor=theme_theme_test(). Foo={{ foo }}
diff --git a/core/modules/system/tests/modules/theme_test/templates/theme_test.template_test.html.twig b/core/modules/system/tests/modules/theme_test/templates/theme_test.template_test.html.twig
new file mode 100644
index 0000000..cde8faa
--- /dev/null
+++ b/core/modules/system/tests/modules/theme_test/templates/theme_test.template_test.html.twig
@@ -0,0 +1,2 @@
+<!-- Output for Theme API test -->
+Fail: Template not overridden.
diff --git a/core/modules/system/tests/modules/theme_test/templates/theme_test.template_test.tpl.php b/core/modules/system/tests/modules/theme_test/templates/theme_test.template_test.tpl.php
deleted file mode 100644
index cde8faa..0000000
--- a/core/modules/system/tests/modules/theme_test/templates/theme_test.template_test.tpl.php
+++ /dev/null
@@ -1,2 +0,0 @@
-<!-- Output for Theme API test -->
-Fail: Template not overridden.
diff --git a/core/modules/system/tests/modules/theme_test/theme_test.inc b/core/modules/system/tests/modules/theme_test/theme_test.inc
index 6cde683..99043fe 100644
--- a/core/modules/system/tests/modules/theme_test/theme_test.inc
+++ b/core/modules/system/tests/modules/theme_test/theme_test.inc
@@ -1,14 +1,13 @@
 <?php
 
 /**
- * Returns HTML for the 'theme_test' theme hook used by tests.
- */
-function theme_theme_test($variables) {
-  return 'Theme hook implementor=theme_theme_test(). Foo=' . $variables['foo'];
-}
-
-/**
- * Preprocesses variables for theme_theme_test().
+ * Prepares variables for theme test templates.
+ *
+ * Default template: theme-test.html.twig.
+ *
+ * @param array $variables
+ *   An associative array containing:
+ *   - foo: Dummy text to display.
  */
 function template_preprocess_theme_test(&$variables) {
   $variables['foo'] = 'template_preprocess_theme_test';
diff --git a/core/modules/system/tests/modules/theme_test/theme_test.module b/core/modules/system/tests/modules/theme_test/theme_test.module
index 137fdfc..07bc084 100644
--- a/core/modules/system/tests/modules/theme_test/theme_test.module
+++ b/core/modules/system/tests/modules/theme_test/theme_test.module
@@ -7,6 +7,7 @@ function theme_test_theme($existing, $type, $theme, $path) {
   $items['theme_test'] = array(
     'file' => 'theme_test.inc',
     'variables' => array('foo' => ''),
+    'template' => 'theme-test',
   );
   $items['theme_test_template_test'] = array(
     'template' => 'theme_test.template_test',
@@ -16,6 +17,7 @@ function theme_test_theme($existing, $type, $theme, $path) {
   );
   $items['theme_test_foo'] = array(
     'variables' => array('foo' => NULL),
+    'template' => 'theme-test-foo',
   );
   return $items;
 }
@@ -164,11 +166,3 @@ function theme_test_preprocess_html(&$variables) {
   $variables['html_attributes']['theme_test_html_attribute'] = 'theme test html attribute value';
   $variables['attributes']['theme_test_body_attribute'] = 'theme test body attribute value';
 }
-
-/**
- * Theme function for testing theme('theme_test_foo').
- */
-function theme_theme_test_foo($variables) {
-  return $variables['foo'];
-}
-