core/core.services.yml                             |  5 ++
 .../Core/Template/Loader/ThemeRegistryLoader.php   | 68 ++++++++++++++++++++
 .../Core/Template/ThemeRegistryNodeVisitor.php     | 72 +++++++++++++---------
 core/modules/image/image.field.inc                 | 31 +---------
 .../image/templates/image-formatter.html.twig      | 18 ++++--
 core/modules/system/components/a/a.html.twig       |  1 +
 core/modules/system/components/a/a.yml             | 30 +++++++++
 core/modules/system/templates/html.html.twig       |  8 ++-
 .../templates/field/image-formatter.html.twig      | 19 ------
 core/themes/classy/templates/layout/html.html.twig | 25 +-------
 .../templates/field/image-formatter.html.twig      | 18 ------
 core/themes/stable/templates/layout/html.html.twig | 47 --------------
 .../navigation/menu-local-tasks.html.twig          | 21 -------
 13 files changed, 169 insertions(+), 194 deletions(-)

diff --git a/core/core.services.yml b/core/core.services.yml
index 5965675..6f7399f 100644
--- a/core/core.services.yml
+++ b/core/core.services.yml
@@ -1547,6 +1547,11 @@ services:
     arguments: ['@app.root', '@module_handler', '@theme_handler']
     tags:
       - { name: twig.loader, priority: 100 }
+  twig.loader.theme_registry:
+    class: Drupal\Core\Template\Loader\ThemeRegistryLoader
+    arguments: ['@theme.registry']
+    tags:
+      - { name: twig.loader, priority: 0 }
   twig.loader.string:
     class: Drupal\Core\Template\Loader\StringLoader
     tags:
diff --git a/core/lib/Drupal/Core/Template/Loader/ThemeRegistryLoader.php b/core/lib/Drupal/Core/Template/Loader/ThemeRegistryLoader.php
new file mode 100644
index 0000000..67a82dd
--- /dev/null
+++ b/core/lib/Drupal/Core/Template/Loader/ThemeRegistryLoader.php
@@ -0,0 +1,68 @@
+<?php
+
+namespace Drupal\Core\Template\Loader;
+
+use Drupal\Core\Theme\Registry;
+
+/**
+ * Loads templates based on information from the Drupal theme registry.
+ *
+ * Allows for template inheritance based on the currently active template.
+ */
+class ThemeRegistryLoader extends \Twig_Loader_Filesystem {
+
+  /**
+   * The theme registry used to determine which template to use.
+   *
+   * @var \Drupal\Core\Theme\Registry
+   */
+  protected $themeRegistry;
+
+  /**
+   * Constructs a new ThemeRegistryLoader object.
+   *
+   * @param \Drupal\Core\Theme\Registry $theme_registry
+   *   The theme registry.
+   */
+  public function __construct(Registry $theme_registry) {
+    $this->themeRegistry = $theme_registry;
+  }
+
+  /**
+   * Finds the path to the requested template.
+   *
+   * @param string $name
+   *   The name of the template to load.
+   * @param bool $throw
+   *   Whether to throw an exception when an error occurs.
+   *
+   * @return string
+   *   The path to the template.
+   *
+   * @throws \Twig_Error_Loader
+   *   Thrown if a template matching $name cannot be found.
+   */
+  protected function findTemplate($name, $throw = TRUE) {
+    // Allow for loading based on the Drupal theme registry.
+    $hook = str_replace('.html.twig', '', strtr($name, '-', '_'));
+    $theme_registry = $this->themeRegistry->getRuntime();
+
+    if ($theme_registry->has($hook)) {
+      $info = $theme_registry->get($hook);
+      if (isset($info['path'])) {
+        $path = $info['path'] . '/' . $name;
+      }
+      elseif (isset($info['template'])) {
+        $path = $info['template'] . '.html.twig';
+      }
+      if (isset($path) && is_file($path)) {
+        return $this->cache[$name] = $path;
+      }
+    }
+
+    if ($throw) {
+      throw new \Twig_Error_Loader(sprintf('Unable to find template "%s" in the Drupal theme registry.', $name));
+    }
+  }
+
+}
diff --git a/core/lib/Drupal/Core/Template/ThemeRegistryNodeVisitor.php b/core/lib/Drupal/Core/Template/ThemeRegistryNodeVisitor.php
index 17c0aa9..8f65170 100644
--- a/core/lib/Drupal/Core/Template/ThemeRegistryNodeVisitor.php
+++ b/core/lib/Drupal/Core/Template/ThemeRegistryNodeVisitor.php
@@ -12,17 +12,22 @@
  *
  * Only updates expressions like:
  * - {% extends "block.html.twig" %}
- * - {% include "block.html.twig" %}
  * - {% extends "block--search-form-block.html.twig" %}
- * - {% include "block--search-form-block.html.twig" %}
  *
  * Does not update expressions like:
+ * - {% extends "@stable/block.twig" %}
  * - {% extends "@stable/block--search-form-block.html.twig" %}
- * - {% include "@stable/block--search-form-block.html.twig" %}
+ * - {% extends "path/to/block.html.twig" %}
  * - {% extends "path/to/block--search-form-block.html.twig" %}
- * - {% include "path/to/block--search-form-block.html.twig" %}
+ *
+ * Ensures that a variant of a theme hook that extends the non-variant does not
+ * skip the current theme's template: if a theme has
+ * block--search-form-block.html.twig that's extending block.html.twig, then it
+ * will use the current theme's block.html.twig if it has it, and not the parent
+ * theme's. This matches the behavior outside of Twig.
  *
  * @see https://www.drupal.org/node/2387069
+ * @see http://twig.sensiolabs.org/doc/tags/extends.html#dynamic-inheritance
  *
  * @see twig_render
  */
@@ -52,27 +57,20 @@ protected function doEnterNode(\Twig_Node $node, \Twig_Environment $env) {
     if ($node instanceof \Twig_Node_Module && $node->hasNode('parent')) {
       $parent = $node->getNode('parent');
       if ($parent && $this->expressionQualifies($parent)) {
-        $current_file = basename($node->getAttribute('filename'));
-        $extended_file = $parent->getAttribute('value');
-        $candidates = ($current_file === $extended_file)
-          ? $this->getCandidateParentTemplates($current_file)
-          : $this->getCandidateTemplates($extended_file);
+        $current_filename = basename($node->getAttribute('filename'));
+        $extended_filename = $parent->getAttribute('value');
+        assert('strpos($current_filename, "/") === FALSE');
+        assert('strpos($extended_filename, "/") === FALSE');
+        $path = dirname($node->getAttribute('filename'));
+        $candidates = ($current_filename === $extended_filename)
+          ? $this->getCandidateParentTemplates($current_filename, $path)
+          : $this->getCandidateTemplates($extended_filename);
         if ($candidates === FALSE) {
           throw new \Twig_Error(sprintf('Template "%s" extends "%s", but no such parent templates exist.', $node->getAttribute('filename'), $parent->getAttribute('value')));
         }
         $node->setNode('parent', $this->getReplacementNode($parent, $candidates));
       }
     }
-    elseif ($node instanceof \Twig_Node_Include) {
-      $include = $node->getNode('expr');
-      if ($include && $this->expressionQualifies($include)) {
-        $candidates = $this->getCandidateTemplates($include->getAttribute('value'));
-        if (empty($candidates)) {
-          throw new \Twig_Error(sprintf('Template "%s" includes "%s", but no such templates exist.', $node->getAttribute('filename'), $include->getAttribute('value')));
-        }
-        $node->setNode('expr', $this->getReplacementNode($include, $candidates));
-      }
-    }
     return $node;
   }
 
@@ -90,6 +88,10 @@ protected function expressionQualifies(\Twig_Node $node) {
     return
       // Only override when extending a single template.
       $node instanceof \Twig_Node_Expression_Constant
+      // Only override when the Module node is not a dynamically created one (as
+      // they are created for Embed nodes).
+      // @see \Twig_TokenParser_Embed
+      && $extended_file !== '__parent__'
       // Only override when the extended template is pointing to a file, i.e
       // when it doesn't include either a path or a namespace.
       && $extended_file === basename($extended_file);
@@ -120,14 +122,14 @@ protected function getReplacementNode(\Twig_Node_Expression_Constant $node, arra
   /**
    * Returns candidate templates based on the theme registry.
    *
-   * @param string $template_filepath
-   *   The full path relative to the Drupal root for this template.
+   * @param string $template_filename
+   *   The template file name.
    *
    * @return string[]
    *   The candidate templates, including the current template.
    */
-  protected function getCandidateTemplates($template_filepath) {
-    $template_filename = basename($template_filepath);
+  protected function getCandidateTemplates($template_filename) {
+    assert('strpos($template_filename, "/") === FALSE');
     $theme_hook = str_replace('.html.twig', '', strtr($template_filename, '-', '_'));
     $info = $this->themeRegistry->getRuntime()->get($theme_hook);
 
@@ -140,22 +142,34 @@ protected function getCandidateTemplates($template_filepath) {
   /**
    * Returns candidate parent templates based on the theme registry.
    *
-   * @param string $template_filepath
-   *   The full path relative to the Drupal root for this template.
+   * @param string $template_filename
+   *   The template file name.
+   * @param string $template_path
+   *   The path relative to the Drupal root for the directory that contains this
+   *   template.
    *
    * @return string[]|false
    *   The candidate parent templates, or FALSE when none exist.
    */
-  protected function getCandidateParentTemplates($template_filepath) {
-    $candidates = $this->getCandidateTemplates($template_filepath);
+  protected function getCandidateParentTemplates($template_filename, $template_path) {
+    assert('strpos($template_filename, "/") === FALSE');
+    $candidates = $this->getCandidateTemplates($template_filename);
 
     // The candidates include the current template too, so there must be >=2.
     if (count($candidates) < 2) {
       return FALSE;
     }
 
-    // Remove the current template.
-    array_shift($candidates);
+    // Remove the current template (and its descendants) from the candidates:
+    // only return ancestors.
+    $reversed_candidates = array_reverse($candidates);
+    for ($i = 0; $i < count($reversed_candidates); $i++) {
+      if ($reversed_candidates[$i]['path'] === $template_path) {
+        array_splice($reversed_candidates, $i);
+        break;
+      }
+    }
+    $candidates = array_reverse($reversed_candidates);
 
     return $candidates;
   }
diff --git a/core/modules/image/image.field.inc b/core/modules/image/image.field.inc
index 1bf4369..03165e2 100644
--- a/core/modules/image/image.field.inc
+++ b/core/modules/image/image.field.inc
@@ -48,34 +48,5 @@ function template_preprocess_image_widget(&$variables) {
  *   - url: An optional \Drupal\Core\Url object.
  */
 function template_preprocess_image_formatter(&$variables) {
-  if ($variables['image_style']) {
-    $variables['image'] = array(
-      '#theme' => 'image_style',
-      '#style_name' => $variables['image_style'],
-    );
-  }
-  else {
-    $variables['image'] = array(
-      '#theme' => 'image',
-    );
-  }
-  $variables['image']['#attributes'] = $variables['item_attributes'];
-
-  $item = $variables['item'];
-
-  // Do not output an empty 'title' attribute.
-  if (Unicode::strlen($item->title) != 0) {
-    $variables['image']['#title'] = $item->title;
-  }
-
-  if (($entity = $item->entity) && empty($item->uri)) {
-    $variables['image']['#uri'] = $entity->getFileUri();
-  }
-  else {
-    $variables['image']['#uri'] = $item->uri;
-  }
-
-  foreach (array('width', 'height', 'alt') as $key) {
-    $variables['image']["#$key"] = $item->$key;
-  }
+  $variables['item_attributes'] = new \Drupal\Core\Template\Attribute($variables['item_attributes']);
 }
diff --git a/core/modules/image/templates/image-formatter.html.twig b/core/modules/image/templates/image-formatter.html.twig
index 63ca3c6..8cbcb25 100644
--- a/core/modules/image/templates/image-formatter.html.twig
+++ b/core/modules/image/templates/image-formatter.html.twig
@@ -13,8 +13,16 @@
  * @ingroup themeable
  */
 #}
-{% if url %}
-  <a href="{{ url }}">{{ image }}</a>
-{% else %}
-  {{ image }}
-{% endif %}
+{% embed "a.html.twig" with { url: url, image: item, attributes: item_attributes, style_name: image_style } only %}
+  {% block content %}
+    {% include "image.html.twig" with {
+      uri: image.entity.uri.value,
+      width: image.width,
+      height: image.height,
+      alt: image.alt,
+      title: image.title,
+      attributes: attributes,
+      style_name: style_name
+    } only %}
+  {% endblock %}
+{% endembed %}
diff --git a/core/modules/system/components/a/a.html.twig b/core/modules/system/components/a/a.html.twig
new file mode 100644
index 0000000..a51949d
--- /dev/null
+++ b/core/modules/system/components/a/a.html.twig
@@ -0,0 +1 @@
+<a href="{{ url }}" class="{{ class }}">{% block content %}{{ text }}{% endblock %}</a>
diff --git a/core/modules/system/components/a/a.yml b/core/modules/system/components/a/a.yml
new file mode 100644
index 0000000..59b6d9b
--- /dev/null
+++ b/core/modules/system/components/a/a.yml
@@ -0,0 +1,30 @@
+label: Anchor
+is_container: true
+variables:
+  url:
+    type: string
+    desc: 'The URL to link to.'
+    example: 'https://www.drupal.org'
+    default: ''
+# This fails because everything expects to have an Attribute object to work with
+# yet Twig templates cannot generate Attribute objects.
+#
+#  attributes:
+#    type: string[]
+#    desc: 'HTML attributes for the anchor.'
+#    example: { href: 'https://www.drupal.org', hreflang: 'en' }
+#    default: []
+  text:
+    type: string
+    example: 'Text'
+    desc: 'The link text.'
+  class:
+    type: string
+    example: 'focusable'
+    desc: 'The class attribute.'
+documentation:
+  purpose: …
+  when: …
+  how: …
+  accessibility: …
+  links: []
diff --git a/core/modules/system/templates/html.html.twig b/core/modules/system/templates/html.html.twig
index 195d82e..e0b73fe 100644
--- a/core/modules/system/templates/html.html.twig
+++ b/core/modules/system/templates/html.html.twig
@@ -38,9 +38,11 @@
       Keyboard navigation/accessibility link to main content section in
       page.html.twig.
     #}
-    <a href="#main-content" class="visually-hidden focusable">
-      {{ 'Skip to main content'|t }}
-    </a>
+    {% include "a.html.twig" with {
+      url: '#main-content',
+      class: 'visually-hidden focusable',
+      text: 'Skip to main content'|t
+    } only %}
     {{ page_top }}
     {{ page }}
     {{ page_bottom }}
diff --git a/core/themes/classy/templates/field/image-formatter.html.twig b/core/themes/classy/templates/field/image-formatter.html.twig
deleted file mode 100644
index 04a98a3..0000000
--- a/core/themes/classy/templates/field/image-formatter.html.twig
+++ /dev/null
@@ -1,19 +0,0 @@
-{#
-/**
- * @file
- * Theme override to display a formatted image field.
- *
- * Available variables:
- * - image: A collection of image data.
- * - image_style: An optional image style.
- * - path: An optional array containing the link 'path' and link 'options'.
- * - url: An optional URL the image can be linked to.
- *
- * @see template_preprocess_image_formatter()
- */
-#}
-{% if url %}
-  <a href="{{ url }}">{{ image }}</a>
-{% else %}
-  {{ image }}
-{% endif %}
diff --git a/core/themes/classy/templates/layout/html.html.twig b/core/themes/classy/templates/layout/html.html.twig
index 8330ccc..ed2cc6e 100644
--- a/core/themes/classy/templates/layout/html.html.twig
+++ b/core/themes/classy/templates/layout/html.html.twig
@@ -1,3 +1,5 @@
+{% extends "html.html.twig" %}
+
 {#
 /**
  * @file
@@ -31,25 +33,4 @@
     db_offline ? 'db-offline',
   ]
 %}
-<!DOCTYPE html>
-<html{{ html_attributes }}>
-  <head>
-    <head-placeholder token="{{ placeholder_token }}">
-    <title>{{ head_title|safe_join(' | ') }}</title>
-    <css-placeholder token="{{ placeholder_token }}">
-    <js-placeholder token="{{ placeholder_token }}">
-  </head>
-  <body{{ attributes.addClass(body_classes) }}>
-    {#
-      Keyboard navigation/accessibility link to main content section in
-      page.html.twig.
-    #}
-    <a href="#main-content" class="visually-hidden focusable skip-link">
-      {{ 'Skip to main content'|t }}
-    </a>
-    {{ page_top }}
-    {{ page }}
-    {{ page_bottom }}
-    <js-bottom-placeholder token="{{ placeholder_token }}">
-  </body>
-</html>
+{% set attributes = attributes.addClass(body_classes) %}
diff --git a/core/themes/stable/templates/field/image-formatter.html.twig b/core/themes/stable/templates/field/image-formatter.html.twig
deleted file mode 100644
index d0390c0..0000000
--- a/core/themes/stable/templates/field/image-formatter.html.twig
+++ /dev/null
@@ -1,18 +0,0 @@
-{#
-/**
- * @file
- * Theme override to display a formatted image field.
- *
- * Available variables:
- * - image: A collection of image data.
- * - image_style: An optional image style.
- * - url: An optional URL the image can be linked to.
- *
- * @see template_preprocess_image_formatter()
- */
-#}
-{% if url %}
-  <a href="{{ url }}">{{ image }}</a>
-{% else %}
-  {{ image }}
-{% endif %}
diff --git a/core/themes/stable/templates/layout/html.html.twig b/core/themes/stable/templates/layout/html.html.twig
deleted file mode 100644
index 5e4f25e..0000000
--- a/core/themes/stable/templates/layout/html.html.twig
+++ /dev/null
@@ -1,47 +0,0 @@
-{#
-/**
- * @file
- * Theme override for the basic structure of a single Drupal page.
- *
- * Variables:
- * - logged_in: A flag indicating if user is logged in.
- * - root_path: The root path of the current page (e.g., node, admin, user).
- * - node_type: The content type for the current node, if the page is a node.
- * - head_title: List of text elements that make up the head_title variable.
- *   May contain or more of the following:
- *   - title: The title of the page.
- *   - name: The name of the site.
- *   - slogan: The slogan of the site.
- * - page_top: Initial rendered markup. This should be printed before 'page'.
- * - page: The rendered page markup.
- * - page_bottom: Closing rendered markup. This variable should be printed after
- *   'page'.
- * - db_offline: A flag indicating if the database is offline.
- * - placeholder_token: The token for generating head, css, js and js-bottom
- *   placeholders.
- *
- * @see template_preprocess_html()
- */
-#}
-<!DOCTYPE html>
-<html{{ html_attributes }}>
-  <head>
-    <head-placeholder token="{{ placeholder_token|raw }}">
-    <title>{{ head_title|safe_join(' | ') }}</title>
-    <css-placeholder token="{{ placeholder_token|raw }}">
-    <js-placeholder token="{{ placeholder_token|raw }}">
-  </head>
-  <body{{ attributes }}>
-    {#
-      Keyboard navigation/accessibility link to main content section in
-      page.html.twig.
-    #}
-    <a href="#main-content" class="visually-hidden focusable">
-      {{ 'Skip to main content'|t }}
-    </a>
-    {{ page_top }}
-    {{ page }}
-    {{ page_bottom }}
-    <js-bottom-placeholder token="{{ placeholder_token|raw }}">
-  </body>
-</html>
diff --git a/core/themes/stable/templates/navigation/menu-local-tasks.html.twig b/core/themes/stable/templates/navigation/menu-local-tasks.html.twig
deleted file mode 100644
index 3874add..0000000
--- a/core/themes/stable/templates/navigation/menu-local-tasks.html.twig
+++ /dev/null
@@ -1,21 +0,0 @@
-{#
-/**
- * @file
- * Theme override to display primary and secondary local tasks.
- *
- * Available variables:
- * - primary: HTML list items representing primary tasks.
- * - secondary: HTML list items representing primary tasks.
- *
- * Each item in these variables (primary and secondary) can be individually
- * themed in menu-local-task.html.twig.
- */
-#}
-{% if primary %}
-  <h2 class="visually-hidden">{{ 'Primary tabs'|t }}</h2>
-  <ul>{{ primary }}</ul>
-{% endif %}
-{% if secondary %}
-  <h2 class="visually-hidden">{{ 'Secondary tabs'|t }}</h2>
-  <ul>{{ secondary }}</ul>
-{% endif %}