diff --git a/core/lib/Drupal/Core/Extension/Extension.php b/core/lib/Drupal/Core/Extension/Extension.php
index 0238714..da8b6e8 100644
--- a/core/lib/Drupal/Core/Extension/Extension.php
+++ b/core/lib/Drupal/Core/Extension/Extension.php
@@ -45,6 +45,13 @@ class Extension implements \Serializable {
   protected $root;
 
   /**
+   * The load method has completed.
+   *
+   * @var bool
+   */
+  protected $loaded = FALSE;
+
+  /**
    * Constructs a new Extension object.
    *
    * @param string $root
@@ -136,8 +143,9 @@ public function getExtensionFilename() {
    *   TRUE if this extension has a main extension file, FALSE otherwise.
    */
   public function load() {
-    if ($this->filename) {
+    if ($this->filename && !$this->loaded) {
       include_once $this->root . '/' . $this->getPath() . '/' . $this->filename;
+      $this->loaded = TRUE;
       return TRUE;
     }
     return FALSE;
diff --git a/core/lib/Drupal/Core/Theme/ThemeInitialization.php b/core/lib/Drupal/Core/Theme/ThemeInitialization.php
index bf58584..27e52a3 100644
--- a/core/lib/Drupal/Core/Theme/ThemeInitialization.php
+++ b/core/lib/Drupal/Core/Theme/ThemeInitialization.php
@@ -127,11 +127,21 @@ public function getActiveThemeByName($theme_name) {
    * {@inheritdoc}
    */
   public function loadActiveTheme(ActiveTheme $active_theme) {
+    // Load the base themes and active theme.
+    foreach ($active_theme->getBaseThemes() as $base) {
+      $base->getExtension()->load();
+      // Include the theme file or the engine.
+      if ($base->getOwner()) {
+        include_once $this->root . '/' . $base->getOwner();
+      }
+    }
+    $active_theme->getExtension()->load();
+    // Include the active theme's owner file too.
+    if ($active_theme->getOwner()) {
+      include_once $this->root . '/' . $active_theme->getOwner();
+    }
     // Initialize the theme.
     if ($theme_engine = $active_theme->getEngine()) {
-      // Include the engine.
-      include_once $this->root . '/' . $active_theme->getOwner();
-
       if (function_exists($theme_engine . '_init')) {
         foreach ($active_theme->getBaseThemes() as $base) {
           call_user_func($theme_engine . '_init', $base->getExtension());
@@ -139,20 +149,6 @@ public function loadActiveTheme(ActiveTheme $active_theme) {
         call_user_func($theme_engine . '_init', $active_theme->getExtension());
       }
     }
-    else {
-      // include non-engine theme files
-      foreach ($active_theme->getBaseThemes() as $base) {
-        // Include the theme file or the engine.
-        if ($base->getOwner()) {
-          include_once $this->root . '/' . $base->getOwner();
-        }
-      }
-      // and our theme gets one too.
-      if ($active_theme->getOwner()) {
-        include_once $this->root . '/' . $active_theme->getOwner();
-      }
-    }
-
     // Always include Twig as the default theme engine.
     include_once $this->root . '/core/themes/engines/twig/twig.engine';
   }
diff --git a/core/themes/engines/twig/twig.engine b/core/themes/engines/twig/twig.engine
index c8f8f49..d76db36 100644
--- a/core/themes/engines/twig/twig.engine
+++ b/core/themes/engines/twig/twig.engine
@@ -26,16 +26,6 @@ function twig_extension() {
 }
 
 /**
- * Includes .theme file from themes.
- *
- * @param \Drupal\Core\Extension\Extension $theme
- *   The theme extension object.
- */
-function twig_init(Extension $theme) {
-  $theme->load();
-}
-
-/**
  * Implements hook_render_template().
  *
  * Renders a Twig template.