diff --git a/core/modules/system/src/Tests/Theme/TwigLoaderTest.php b/core/modules/system/src/Tests/Theme/TwigLoaderTest.php
new file mode 100644
index 0000000..91de1e4
--- /dev/null
+++ b/core/modules/system/src/Tests/Theme/TwigLoaderTest.php
@@ -0,0 +1,39 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\system\Tests\Theme\TwigLoaderTest.
+ */
+
+namespace Drupal\system\Tests\Theme;
+
+use Drupal\simpletest\WebTestBase;
+
+/**
+ * Tests adding Twig loaders.
+ *
+ * @group Theme
+ */
+class TwigLoaderTest extends WebTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  public static $modules = ['twig_loader_test'];
+
+  /**
+   * Tests adding an additional twig loader to the loader chain.
+   */
+  public function testTwigLoaderAddition() {
+    $environment = \Drupal::service('twig');
+
+    $template = $environment->loadTemplate('kittens');
+    $this->assertEqual($template->render(array()), 'kittens', 'Passing "kittens" to the custom Twig loader returns "kittens".');
+
+    $template = $environment->loadTemplate('meow');
+    $this->assertEqual($template->render(array()), 'cats', 'Passing something other than "kittens" to the custom Twig loader returns "cats".');
+  }
+
+}
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..589632b
--- /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();
+    \Drupal::service('theme_handler')->install(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.
+    $this->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');
+    $this->assertText('This line is from test_theme_twig_registry_loader/templates/twig-registry-loader-test-include.html.twig');
+  }
+
+}
diff --git a/core/modules/system/tests/modules/twig_loader_test/src/Loader/TestLoader.php b/core/modules/system/tests/modules/twig_loader_test/src/Loader/TestLoader.php
new file mode 100644
index 0000000..9f32969
--- /dev/null
+++ b/core/modules/system/tests/modules/twig_loader_test/src/Loader/TestLoader.php
@@ -0,0 +1,50 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\twig_loader_test\Loader\TestLoader.
+ */
+
+namespace Drupal\twig_loader_test\Loader;
+
+use Drupal\Core\Template\Loader;
+
+/**
+ * A test Twig loader.
+ */
+class TestLoader implements \Twig_LoaderInterface, \Twig_ExistsLoaderInterface {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getSource($name) {
+    if ($name == 'kittens') {
+      return $name;
+    }
+    else {
+      return 'cats';
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function exists($name) {
+    return TRUE;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getCacheKey($name) {
+    return $name;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function isFresh($name, $time) {
+    return TRUE;
+  }
+
+}
diff --git a/core/modules/system/tests/modules/twig_loader_test/twig_loader_test.info.yml b/core/modules/system/tests/modules/twig_loader_test/twig_loader_test.info.yml
new file mode 100644
index 0000000..cf04aae
--- /dev/null
+++ b/core/modules/system/tests/modules/twig_loader_test/twig_loader_test.info.yml
@@ -0,0 +1,6 @@
+name: 'Twig Loader Test'
+type: module
+description: 'Support module for testing adding Twig loaders.'
+package: Testing
+version: VERSION
+core: 8.x
diff --git a/core/modules/system/tests/modules/twig_loader_test/twig_loader_test.services.yml b/core/modules/system/tests/modules/twig_loader_test/twig_loader_test.services.yml
new file mode 100644
index 0000000..1b19948
--- /dev/null
+++ b/core/modules/system/tests/modules/twig_loader_test/twig_loader_test.services.yml
@@ -0,0 +1,5 @@
+services:
+  twig_loader_test.twig.test_loader:
+    class: Drupal\twig_loader_test\Loader\TestLoader
+    tags:
+      - { name: twig.loader }
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 96fcb67..2bf5394 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
@@ -58,4 +58,11 @@ public function fileUrlRender() {
     );
   }
 
+  /**
+   * 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-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..e18e68e
--- /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..438fd2b
--- /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/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..a3723b5
--- /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/twig_theme_test.module b/core/modules/system/tests/modules/twig_theme_test/twig_theme_test.module
index 4a415cd..e42b020 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 +19,15 @@ function twig_theme_test_theme($existing, $type, $theme, $path) {
     'variables' => array(),
     'template' => 'twig_namespace_test',
   );
+  $items['twig_registry_loader_test'] = array(
+    'variables' => array(),
+  );
+  $items['twig_registry_loader_test_include'] = array(
+    'variables' => array(),
+  );
+  $items['twig_registry_loader_test_extend'] = array(
+    'variables' => array(),
+  );
   $items['twig_raw_test'] = array(
     'variables' => array('script' => ''),
   );
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 d2abf91..4d30bb9 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
@@ -32,3 +32,10 @@ twig_theme_test_file_url:
     _controller: '\Drupal\twig_theme_test\TwigThemeTestController::fileUrlRender'
   requirements:
     _access: 'TRUE'
+
+twig_theme_test_registry_loader:
+  path: '/twig-theme-test/registry-loader'
+  defaults:
+    _controller: '\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..e1a9eee
--- /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..3a1f395
--- /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
diff --git a/core/tests/Drupal/Tests/Core/DependencyInjection/Compiler/TaggedHandlersPassTest.php b/core/tests/Drupal/Tests/Core/DependencyInjection/Compiler/TaggedHandlersPassTest.php
index dd03a1f..db6f39f 100644
--- a/core/tests/Drupal/Tests/Core/DependencyInjection/Compiler/TaggedHandlersPassTest.php
+++ b/core/tests/Drupal/Tests/Core/DependencyInjection/Compiler/TaggedHandlersPassTest.php
@@ -42,6 +42,25 @@ public function testProcessNoConsumers() {
   }
 
   /**
+   * Tests a required consumer with no handlers.
+   *
+   * @expectedException \Symfony\Component\DependencyInjection\Exception\LogicException
+   * @expectedExceptionMessage At least one service tagged with 'consumer_id' is required.
+   * @covers ::process
+   */
+  public function testProcessRequiredHandlers() {
+    $container = $this->buildContainer();
+    $container
+      ->register('consumer_id', __NAMESPACE__ . '\ValidConsumer')
+      ->addTag('service_collector', array(
+        'required' => TRUE,
+      ));
+
+    $handler_pass = new TaggedHandlersPass();
+    $handler_pass->process($container);
+  }
+
+  /**
    * Tests consumer with missing interface in non-production environment.
    *
    * @expectedException \Symfony\Component\DependencyInjection\Exception\LogicException