commit e5123f52b507b19c9fc7520aa9e4079f14eb5a4d
Author: Joel Pittet <joel@pittet.ca>
Date:   Wed Sep 3 20:45:32 2014 -0700

    add back default_attributes clone

diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index 1f28b53..ac79094 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -212,6 +212,8 @@ function list_themes($refresh = FALSE) {
  * @see template_preprocess()
  */
 function _theme($hook, $variables = array()) {
+  static $default_attributes;
+
   $module_handler = \Drupal::moduleHandler();
   $active_theme = \Drupal::theme()->getActiveTheme();
 
@@ -412,8 +414,17 @@ function _theme($hook, $variables = array()) {
       template_preprocess($default_template_variables, $hook, $info);
       $variables += $default_template_variables;
     }
+    if (!isset($default_attributes)) {
+      $default_attributes = new Attribute();
+    }
     if (isset($variables['attributes']) && !($variables['attributes'] instanceof Attribute)) {
-      $variables['attributes'] = new Attribute($variables['attributes']);
+      if ($variables['attributes']) {
+        $variables['attributes'] = new Attribute($variables['attributes']);
+      }
+      else {
+        // Create empty attributes.
+        $variables['attributes'] = clone $default_attributes;
+      }
     }
 
     // Render the output using the template file.
@@ -2112,6 +2123,11 @@ function template_preprocess_field(&$variables, $hook) {
     $variables['title_attributes']['class'][] = 'visually-hidden';
   }
 
+  static $default_attributes;
+  if (!isset($default_attributes)) {
+    $default_attributes = new Attribute;
+  }
+
   // Modules (e.g., rdf.module) can add field item attributes (to
   // $item->_attributes) within hook_entity_prepare_view(). Some field
   // formatters move those attributes into some nested formatter-specific
@@ -2120,11 +2136,11 @@ function template_preprocess_field(&$variables, $hook) {
   // formatters leave them within $element['#items'][$delta]['_attributes'] to
   // be rendered on the item wrappers provided by field.html.twig.
   foreach ($variables['items'] as $delta => $item) {
-    $variables['item_attributes'][$delta] = !empty($element['#items'][$delta]->_attributes) ? new Attribute($element['#items'][$delta]->_attributes) : new Attribute();
+    $variables['item_attributes'][$delta] = !empty($element['#items'][$delta]->_attributes) ? new Attribute($element['#items'][$delta]->_attributes) : clone $default_attributes;
   }
 
-  $variables['title_attributes'] = isset($variables['title_attributes']) ? new Attribute($variables['title_attributes']) : new Attribute();
-  $variables['content_attributes'] = isset($variables['content_attributes']) ? new Attribute($variables['content_attributes']) : new Attribute();
+  $variables['title_attributes'] = isset($variables['title_attributes']) ? new Attribute($variables['title_attributes']) : clone $default_attributes;
+  $variables['content_attributes'] = isset($variables['content_attributes']) ? new Attribute($variables['content_attributes']) : clone $default_attributes;
 }
 
 /**