commit 4b1df0af377decbde72f7ce8973c93ff8848b378
Author: David Mignot <david.mig@gmail.com>
Date:   Thu May 9 10:40:50 2013 +0200

    1898034-84

diff --git a/core/modules/block/block.module b/core/modules/block/block.module
index 0b1ebc9..c74be28 100644
--- a/core/modules/block/block.module
+++ b/core/modules/block/block.module
@@ -6,7 +6,6 @@
  */
 
 use Drupal\Component\Plugin\Exception\PluginException;
-use Drupal\Core\Template\Attribute;
 
 /**
  * Denotes that a block is not enabled in any region and should not be shown.
@@ -547,9 +546,7 @@ function template_preprocess_block(&$variables) {
   // Create the $content variable that templates expect.
   $variables['content'] = $variables['elements']['#children'];
 
-  $variables['attributes'] = new Attribute;
-  $variables['attributes']['class'] = array();
-
+  // Add default class for block content.
   $variables['attributes']['class'][] = drupal_html_class('block-' . $variables['block']->module);
 
   $variables['theme_hook_suggestions'][] = 'block__' . $variables['block']->module;
diff --git a/core/modules/block/templates/block.html.twig b/core/modules/block/templates/block.html.twig
index f135bda..5b78ae1 100644
--- a/core/modules/block/templates/block.html.twig
+++ b/core/modules/block/templates/block.html.twig
@@ -28,14 +28,14 @@
  */
  @todo Remove the div around content as per http://drupal.org/node/1972122.
 #}
-<div class="block {{ attributes.class }}"{{ attributes }}>
+<div{{ attributes }}>
   {{ title_prefix }}
   {% if label %}
     <h2{{ title_attributes }}>{{ label }}</h2>
   {% endif %}
   {{ title_suffix }}
 
-  <div class="content {{ content_attributes.class }}"{{ content_attributes }}>
+  <div{{ content_attributes }}>
    {{ content }}
   </div>
 </div>