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..6627891
--- /dev/null
+++ b/core/modules/system/tests/modules/theme_test/templates/theme-test-foo.html.twig
@@ -0,0 +1,15 @@
+{#
+/**
+ * @file
+ * Default theme implementation for the 'theme_test' theme hook used by tests.
+ *
+ * Available variables:
+ *  - foo: dummy text
+ *
+ * @see template_preprocess()
+ * @see template_preprocess_theme_test_foo()
+ *
+ * @ingroup themeable
+ */
+#}
+{{ foo }}
\ No newline at end of file
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..b388c08
--- /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_foo' theme hook used by tests.
+ *
+ * Available variables:
+ *  - foo: dummy text
+ *
+ * @see template_preprocess()
+ * @see template_preprocess_theme_test()
+ *
+ * @ingroup themeable
+ */
+#}
+Theme hook implementor=theme_theme_test(). Foo={{ foo }}
\ No newline at end of file
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..038f462 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().
+ * Preprocesses variables for theme test templates.
+ *
+ * Default template: theme-test.html.twig
+ *
+ * @param array $variables
+ *   An associative array containing:
+ *   - foo: dummy text
  */
 function template_preprocess_theme_test(&$variables) {
   $variables['foo'] = 'template_preprocess_theme_test';
diff --git a/core/modules/system/tests/modules/theme_test/theme_test.module b/core/modules/system/tests/modules/theme_test/theme_test.module
index 137fdfc..5cfbcc6 100644
--- a/core/modules/system/tests/modules/theme_test/theme_test.module
+++ b/core/modules/system/tests/modules/theme_test/theme_test.module
@@ -166,9 +166,14 @@ function theme_test_preprocess_html(&$variables) {
 }
 
 /**
- * Theme function for testing theme('theme_test_foo').
+ * Preprocesses variables for theme test "foo" templates.
+ *
+ * Default template: theme-test-foo.html.twig
+ *
+ * @param array $variables
+ *   An associative array containing:
+ *   - foo: dummy text
  */
-function theme_theme_test_foo($variables) {
-  return $variables['foo'];
+function template_preprocess_theme_test_foo(&$variables) {
+    // nothing to do here right now
 }
-