diff --git a/core/core.services.yml b/core/core.services.yml
index 07e1053..1d32b46 100644
--- a/core/core.services.yml
+++ b/core/core.services.yml
@@ -391,9 +391,6 @@ services:
   controller.ajax:
     class: Drupal\Core\Controller\AjaxController
     arguments: ['@controller_resolver']
-  controller.form:
-    class: Drupal\Core\Controller\FormController
-    arguments: ['@controller_resolver', '@service_container']
   controller.entityform:
     class: Drupal\Core\Entity\HtmlEntityFormController
     arguments: ['@controller_resolver', '@service_container', '@entity.manager']
diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index a08c9b4..9eb1c96 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -2069,6 +2069,9 @@ function template_preprocess_html(&$variables) {
 
   // Compile a list of classes that are going to be applied to the body element.
   // This allows advanced theming based on context (home page, node of certain type, etc.).
+
+  // $variables['attributes'] is an Attribute object, not an array, so we need
+  // a temporary variable to avoid reference assignment issues.
   $body_classes = $variables['attributes']['class'];
   $body_classes[] = 'html';
   // Add a class that tells us whether we're on the front page or not.
diff --git a/core/lib/Drupal/Core/Controller/AjaxController.php b/core/lib/Drupal/Core/Controller/AjaxController.php
index 8d2b83c..5ed7fb5 100644
--- a/core/lib/Drupal/Core/Controller/AjaxController.php
+++ b/core/lib/Drupal/Core/Controller/AjaxController.php
@@ -63,7 +63,7 @@ public function content(Request $request, $_content) {
       $content = $content->getContent();
     }
 
-    // A page callback could return a render array or a string.
+    // Most controllers return a render array, but some return a string.
     if (!is_array($content)) {
       $content = array(
         '#markup' => $content,
diff --git a/core/lib/Drupal/Core/Controller/DialogController.php b/core/lib/Drupal/Core/Controller/DialogController.php
index 2263f4e..338cac4 100644
--- a/core/lib/Drupal/Core/Controller/DialogController.php
+++ b/core/lib/Drupal/Core/Controller/DialogController.php
@@ -84,6 +84,7 @@ public function dialog(Request $request, $_content, $modal = FALSE) {
       $page_content = $page_content->getContent();
     }
 
+    // Most controllers return a render array, but some return a string.
     if (!is_array($page_content)) {
       $page_content = array(
         '#markup' => $page_content,