diff --git a/core/modules/system/src/Tests/Theme/TwigRegistryLoaderTest.php b/core/modules/system/src/Tests/Theme/TwigRegistryLoaderTest.php
new file mode 100644
index 0000000..e62497b
--- /dev/null
+++ b/core/modules/system/src/Tests/Theme/TwigRegistryLoaderTest.php
@@ -0,0 +1,70 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\system\Tests\Theme\TwigRegistryLoaderTest.
+ */
+
+namespace Drupal\system\Tests\Theme;
+
+use Drupal\simpletest\WebTestBase;
+
+/**
+ * Tests Twig registry loader.
+ *
+ * @group Theme
+ */
+class TwigRegistryLoaderTest extends WebTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  public static $modules = array('twig_theme_test', 'block');
+
+  /**
+   * @var \Drupal\Core\Template\TwigEnvironment
+   */
+  protected $twig;
+
+  protected function setUp() {
+    parent::setUp();
+    theme_enable(array('test_theme_twig_registry_loader'));
+    $this->twig = \Drupal::service('twig');
+  }
+
+  /**
+   * Checks to see if a value is a twig template.
+   */
+  public function assertTwigTemplate($value, $message = '', $group = 'Other') {
+    $this->assertTrue($value instanceof \Twig_Template, $message, $group);
+  }
+
+  /**
+   * Tests template discovery using the Drupal theme registry.
+   */
+  public function testTemplateDiscovery() {
+    $this->assertTwigTemplate($this->twig->resolveTemplate('block.html.twig'), 'Found block.html.twig in block module.');
+  }
+
+  /**
+   * Tests template extension and includes using the Drupal theme registry.
+   */
+  public function testTwigNamespaces() {
+    // Test the module-provided extend and insert templates.
+    $this->drupalGet('twig-theme-test/registry-loader');
+    $this->assertText('This line is from twig_theme_test/templates/twig_registry_loader_test_extend.html.twig');
+    $this->assertText('This line is from twig_theme_test/templates/twig_registry_loader_test_include.html.twig');
+
+    // Enable a theme that overrides the extend and insert templates to ensure
+    // they are picked up by the registry loader.
+    \Drupal::config('system.theme')
+      ->set('default', 'test_theme_twig_registry_loader')
+      ->save();
+    $this->drupalGet('twig-theme-test/registry-loader');
+    $this->assertText('This line is from test_theme_twig_registry_loader/templates/twig_registry_loader_test_extend.html.twig');
+    $this->assertText('This line is from test_theme_twig_registry_loader/templates/twig_registry_loader_test_include.html.twig');
+  }
+
+}
diff --git a/core/modules/system/tests/modules/twig_theme_test/src/TwigThemeTestController.php b/core/modules/system/tests/modules/twig_theme_test/src/TwigThemeTestController.php
index d454e5f..842a8e8 100644
--- a/core/modules/system/tests/modules/twig_theme_test/src/TwigThemeTestController.php
+++ b/core/modules/system/tests/modules/twig_theme_test/src/TwigThemeTestController.php
@@ -37,4 +37,11 @@ public function urlGeneratorRender() {
     );
   }

+  /**
+   * Menu callback for testing the Twig registry loader.
+   */
+  public function registryLoaderRender() {
+    return array('#theme' => 'twig_registry_loader_test');
+  }
+
 }
diff --git a/core/modules/system/tests/modules/twig_theme_test/templates/twig_registry_loader_test.html.twig b/core/modules/system/tests/modules/twig_theme_test/templates/twig_registry_loader_test.html.twig
new file mode 100644
index 0000000..8776f5e
--- /dev/null
+++ b/core/modules/system/tests/modules/twig_theme_test/templates/twig_registry_loader_test.html.twig
@@ -0,0 +1,5 @@
+{% extends "twig_registry_loader_test_extend.html.twig" %}
+
+{% block content %}
+  {% include "twig_registry_loader_test_include.html.twig" %}
+{% endblock %}
diff --git a/core/modules/system/tests/modules/twig_theme_test/templates/twig_registry_loader_test_extend.html.twig b/core/modules/system/tests/modules/twig_theme_test/templates/twig_registry_loader_test_extend.html.twig
new file mode 100644
index 0000000..a5a5af1
--- /dev/null
+++ b/core/modules/system/tests/modules/twig_theme_test/templates/twig_registry_loader_test_extend.html.twig
@@ -0,0 +1,5 @@
+This line is from twig_theme_test/templates/twig_registry_loader_test_extend.html.twig
+
+{% block content %}
+This text is in a block.
+{% endblock %}
diff --git a/core/modules/system/tests/modules/twig_theme_test/templates/twig_registry_loader_test_include.html.twig b/core/modules/system/tests/modules/twig_theme_test/templates/twig_registry_loader_test_include.html.twig
new file mode 100644
index 0000000..bb8c056
--- /dev/null
+++ b/core/modules/system/tests/modules/twig_theme_test/templates/twig_registry_loader_test_include.html.twig
@@ -0,0 +1 @@
+This line is from twig_theme_test/templates/twig_registry_loader_test_include.html.twig
diff --git a/core/modules/system/tests/modules/twig_theme_test/twig_theme_test.module b/core/modules/system/tests/modules/twig_theme_test/twig_theme_test.module
index 0497c7b..046e994 100644
--- a/core/modules/system/tests/modules/twig_theme_test/twig_theme_test.module
+++ b/core/modules/system/tests/modules/twig_theme_test/twig_theme_test.module
@@ -19,6 +19,18 @@ function twig_theme_test_theme($existing, $type, $theme, $path) {
     'variables' => array(),
     'template' => 'twig_namespace_test',
   );
+  $items['twig_registry_loader_test'] = array(
+    'variables' => array(),
+    'template' => 'twig_registry_loader_test',
+  );
+  $items['twig_registry_loader_test_include'] = array(
+    'variables' => array(),
+    'template' => 'twig_registry_loader_test_include',
+  );
+  $items['twig_registry_loader_test_extend'] = array(
+    'variables' => array(),
+    'template' => 'twig_registry_loader_test_extend',
+  );
   $items['twig_raw_test'] = array(
     'variables' => array('script' => ''),
     'template' => 'twig-raw-test',
diff --git a/core/modules/system/tests/modules/twig_theme_test/twig_theme_test.routing.yml b/core/modules/system/tests/modules/twig_theme_test/twig_theme_test.routing.yml
index 75b9bbb..77f1bf3 100644
--- a/core/modules/system/tests/modules/twig_theme_test/twig_theme_test.routing.yml
+++ b/core/modules/system/tests/modules/twig_theme_test/twig_theme_test.routing.yml
@@ -18,3 +18,10 @@ twig_theme_test_url_generator:
     _content: '\Drupal\twig_theme_test\TwigThemeTestController::urlGeneratorRender'
   requirements:
     _access: 'TRUE'
+
+twig_theme_test.registry_loader:
+  path: '/twig-theme-test/registry-loader'
+  defaults:
+    _content: '\Drupal\twig_theme_test\TwigThemeTestController::registryLoaderRender'
+  requirements:
+    _access: 'TRUE'
diff --git a/core/modules/system/tests/themes/test_theme_twig_registry_loader/templates/twig_registry_loader_test_extend.html.twig b/core/modules/system/tests/themes/test_theme_twig_registry_loader/templates/twig_registry_loader_test_extend.html.twig
new file mode 100644
index 0000000..7d5cb0e
--- /dev/null
+++ b/core/modules/system/tests/themes/test_theme_twig_registry_loader/templates/twig_registry_loader_test_extend.html.twig
@@ -0,0 +1,5 @@
+This line is from test_theme_twig_registry_loader/templates/twig_registry_loader_test_extend.html.twig
+
+{% block content %}
+This text is in a block.
+{% endblock %}
diff --git a/core/modules/system/tests/themes/test_theme_twig_registry_loader/templates/twig_registry_loader_test_include.html.twig b/core/modules/system/tests/themes/test_theme_twig_registry_loader/templates/twig_registry_loader_test_include.html.twig
new file mode 100644
index 0000000..7097adf
--- /dev/null
+++ b/core/modules/system/tests/themes/test_theme_twig_registry_loader/templates/twig_registry_loader_test_include.html.twig
@@ -0,0 +1 @@
+This line is from test_theme_twig_registry_loader/templates/twig_registry_loader_test_include.html.twig
diff --git a/core/modules/system/tests/themes/test_theme_twig_registry_loader/test_theme_twig_registry_loader.info.yml b/core/modules/system/tests/themes/test_theme_twig_registry_loader/test_theme_twig_registry_loader.info.yml
new file mode 100644
index 0000000..9ee01c5
--- /dev/null
+++ b/core/modules/system/tests/themes/test_theme_twig_registry_loader/test_theme_twig_registry_loader.info.yml
@@ -0,0 +1,5 @@
+name: 'Twig registry loader test'
+type: theme
+description: 'Support module for Twig registry loader testing.'
+version: VERSION
+core: 8.x