Update to Drupal 8.1.8. For more information, see https://www.drupal.org/project/drupal/releases/8.1.8

core/lib/Drupal/Core/Render/AttachmentsResponseProcessorInterface.php

@@ -25,7 +25,7 @@ interface AttachmentsResponseProcessorInterface {
    * $build['#attached']['library'][] = [
    *   'library' => ['core/jquery']
    * ];
-   * $build['#attached']['http_header'][] = [
+   * $build['#attached']['http_header'] = [
    *   ['Content-Type', 'application/rss+xml; charset=utf-8'],
    * ];
    * @endcode

core/lib/Drupal/Core/Render/Renderer.php

@@ -636,8 +636,33 @@ class Renderer implements RendererInterface {
       return FALSE;
     }
 
-    foreach (array_keys($elements['#attached']['placeholders']) as $placeholder) {
-      $elements = $this->renderPlaceholder($placeholder, $elements);
+    // The 'status messages' placeholder needs to be special cased, because it
+    // depends on global state that can be modified when other placeholders are
+    // being rendered: any code can add messages to render.
+    // This violates the principle that each lazy builder must be able to render
+    // itself in isolation, and therefore in any order. However, we cannot
+    // change the way drupal_set_message() works in the Drupal 8 cycle. So we
+    // have to accommodate its special needs.
+    // Allowing placeholders to be rendered in a particular order (in this case:
+    // last) would violate this isolation principle. Thus a monopoly is granted
+    // to this one special case, with this hard-coded solution.
+    // @see \Drupal\Core\Render\Element\StatusMessages
+    // @see https://www.drupal.org/node/2712935#comment-11368923
+
+    // First render all placeholders except 'status messages' placeholders.
+    $message_placeholders = [];
+    foreach ($elements['#attached']['placeholders'] as $placeholder => $placeholder_element) {
+      if (isset($placeholder_element['#lazy_builder']) && $placeholder_element['#lazy_builder'][0] === 'Drupal\Core\Render\Element\StatusMessages::renderMessages') {
+        $message_placeholders[] = $placeholder;
+      }
+      else {
+        $elements = $this->renderPlaceholder($placeholder, $elements);
+      }
+    }
+
+    // Then render 'status messages' placeholders.
+    foreach ($message_placeholders as $message_placeholder) {
+      $elements = $this->renderPlaceholder($message_placeholder, $elements);
     }
 
     return TRUE;

core/lib/Drupal/Core/Render/theme.api.php

@@ -788,17 +788,17 @@ function hook_render_template($template_file, $variables) {
  * A module may implement this hook in order to alter the element type defaults
  * defined by a module.
  *
- * @param array $types
+ * @param array $info
  *   An associative array with structure identical to that of the return value
  *   of \Drupal\Core\Render\ElementInfoManagerInterface::getInfo().
  *
  * @see \Drupal\Core\Render\ElementInfoManager
  * @see \Drupal\Core\Render\Element\ElementInterface
  */
-function hook_element_info_alter(array &$types) {
+function hook_element_info_alter(array &$info) {
   // Decrease the default size of textfields.
-  if (isset($types['textfield']['#size'])) {
-    $types['textfield']['#size'] = 40;
+  if (isset($info['textfield']['#size'])) {
+    $info['textfield']['#size'] = 40;
   }
 }