Issue #2329853 by Cottser: Move miscellaneous theme.inc classes from preprocess to templates.

core/includes/theme.inc

@@ -1493,7 +1493,6 @@ function template_preprocess_feed_icon(&$variables) {
     '#height' => 16,
     '#alt' => $text,
   );
-  $variables['attributes']['class'] = array('feed-icon');
   // Stripping tags because that's what l() used to do.
   $variables['attributes']['title'] = strip_tags($text);
 }

core/modules/system/templates/feed-icon.html.twig

@@ -15,4 +15,4 @@
  * @ingroup themeable
  */
 #}
-<a href="{{ url }}"{{ attributes }}>{{ icon }}</a>
+<a href="{{ url }}"{{ attributes.addClass('feed-icon') }}>{{ icon }}</a>

core/modules/system/templates/status-messages.html.twig

@@ -24,7 +24,13 @@
  */
 #}
 {% for type, messages in message_list %}
-  <div class="messages messages--{{ type }}" role="contentinfo" aria-label="{{ status_headings[type] }}">
+  {%
+    set classes = [
+      'messages',
+      'messages--' ~ type,
+    ]
+  %}
+  <div class="{{ attributes.addClass(classes).class }}" role="contentinfo" aria-label="{{ status_headings[type] }}">
     {% if type == 'error' %}
       <div role="alert">
     {% endif %}