diff --git a/core/includes/install.core.inc b/core/includes/install.core.inc
index 6514db8..85dc45c 100644
--- a/core/includes/install.core.inc
+++ b/core/includes/install.core.inc
@@ -9,7 +9,6 @@
 
 use Symfony\Component\DependencyInjection\ContainerBuilder;
 use Symfony\Component\DependencyInjection\Reference;
-use Symfony\Component\DependencyInjection\Definition;
 use Symfony\Component\HttpFoundation\Request;
 use Symfony\Component\HttpFoundation\Response;
 
diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index 2813583..2f98018 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -1640,53 +1640,14 @@ function theme_datetime($variables) {
 }
 
 /**
- * Returns HTML for status and/or error messages, grouped by type.
+ * Prepares variables for status message templates.
  *
- * An invisible heading identifies the messages for assistive technology.
- * Sighted users see a colored box. See http://www.w3.org/TR/WCAG-TECHS/H69.html
- * for info.
- *
- * @param $variables
- *   An associative array containing:
- *   - display: (optional) Set to 'status' or 'error' to display only messages
- *     of that type.
- */
-function theme_status_messages($variables) {
-  $display = $variables['display'];
-  $output = '';
-
-  $status_heading = array(
-    'status' => t('Status message'),
-    'error' => t('Error message'),
-    'warning' => t('Warning message'),
-  );
-  foreach (drupal_get_messages($display) as $type => $messages) {
-    $output .= "<div class=\"messages $type\">\n";
-    if (!empty($status_heading[$type])) {
-      $output .= '<h2 class="element-invisible">' . $status_heading[$type] . "</h2>\n";
-    }
-    if (count($messages) > 1) {
-      $output .= " <ul>\n";
-      foreach ($messages as $message) {
-        $output .= '  <li>' . $message . "</li>\n";
-      }
-      $output .= " </ul>\n";
-    }
-    else {
-      $output .= $messages[0];
-    }
-    $output .= "</div>\n";
-  }
-  return $output;
-}
-
-/**
- * Prepares variables for theme_status_messages().
+ * Default template: status-messages.html.twig.
  *
  * @param array $variables
  *   An associative array containing:
- *   - display: (optional) Set to 'status' or 'error' to display only messages
- *     of that type.
+ *   - display: (optional) Set to e.g. 'status' or 'error' to display only
+ *     messages of that type.
  */
 function template_preprocess_status_messages(&$variables) {
   $variables['messages_display'] = drupal_get_messages($variables['display']);
diff --git a/core/modules/system/templates/status-messages.html.twig b/core/modules/system/templates/status-messages.html.twig
index dbcf5fb..347b895 100644
--- a/core/modules/system/templates/status-messages.html.twig
+++ b/core/modules/system/templates/status-messages.html.twig
@@ -8,9 +8,8 @@
  * for info.
  *
  * Available variables:
- * - messages_display: List of messages of all statuses which attempts to be
- *   displayed.
- * - status_heading: List of all status types.
+ * - messages_display: List of messages to be displayed, grouped by type.
+ * - status_headings: List of all status types.
  *
  * @see template_preprocess()
  * @see template_preprocess_status_messages()
@@ -18,11 +17,10 @@
  * @ingroup themeable
  */
 #}
-
 {% for type, messages in messages_display %}
   <div class="messages {{ type }}">
-    {% if type in status_heading %}
-      <h2 class="element-invisible">{{ status_heading[type] }}</h2>
+    {% if type in status_headings %}
+      <h2 class="element-invisible">{{ status_headings[type] }}</h2>
     {% endif %}
     {% if messages|length > 1 %}
       <ul>
@@ -31,7 +29,7 @@
         {% endfor %}
       </ul>
     {% else %}
-      {{ messages[0] }}
+      {{ messages.0 }}
     {% endif %}
   </div>
 {% endfor %}