diff -u b/core/modules/block/lib/Drupal/block/BlockViewBuilder.php b/core/modules/block/lib/Drupal/block/BlockViewBuilder.php
--- b/core/modules/block/lib/Drupal/block/BlockViewBuilder.php
+++ b/core/modules/block/lib/Drupal/block/BlockViewBuilder.php
@@ -7,7 +7,6 @@
 
 namespace Drupal\block;
 
-use Drupal\Component\Utility\NestedArray;
 use Drupal\Core\Entity\EntityViewBuilderInterface;
 use Drupal\Core\Entity\EntityInterface;
 
@@ -74,13 +73,11 @@
         // #attributes and #contextual_links is information about the *entire*
         // block. Therefore, we must move these properties from $content and
         // merge them into the top-level element.
-        if (isset($content['#attributes'])) {
-          $build[$key]['#attributes'] = NestedArray::mergeDeep($build[$key]['#attributes'], $content['#attributes']);
-          unset($content['#attributes']);
-        }
-        if (isset($content['#contextual_links'])) {
-          $build[$key]['#contextual_links'] += $content['#contextual_links'];
-          unset($content['#contextual_links']);
+        foreach (array('#attributes', '#contextual_links') as $property) {
+          if (isset($content[$property])) {
+            $build[$key][$property] += $content[$property];
+            unset($content[$property]);
+          }
         }
         $build[$key]['content'] = $content;
       }
diff -u b/core/modules/block/lib/Drupal/block/Tests/BlockHtmlTest.php b/core/modules/block/lib/Drupal/block/Tests/BlockHtmlTest.php
--- b/core/modules/block/lib/Drupal/block/Tests/BlockHtmlTest.php
+++ b/core/modules/block/lib/Drupal/block/Tests/BlockHtmlTest.php
@@ -2,7 +2,7 @@
 
 /**
  * @file
- * Definition of Drupal\block\Tests\BlockHtmlIdTest.
+ * Definition of Drupal\block\Tests\BlockHtmlTest.
  */
 
 namespace Drupal\block\Tests;
@@ -12,7 +12,7 @@
 /**
  * Tests block HTML ID validity.
  */
-class BlockHtmlIdTest extends WebTestBase {
+class BlockHtmlTest extends WebTestBase {
 
   /**
    * Modules to enable.
diff -u b/core/modules/block/tests/modules/block_test/lib/Drupal/block_test/Plugin/Block/TestHtmlBlock.php b/core/modules/block/tests/modules/block_test/lib/Drupal/block_test/Plugin/Block/TestHtmlBlock.php
--- b/core/modules/block/tests/modules/block_test/lib/Drupal/block_test/Plugin/Block/TestHtmlBlock.php
+++ b/core/modules/block/tests/modules/block_test/lib/Drupal/block_test/Plugin/Block/TestHtmlBlock.php
@@ -7,6 +7,8 @@
 
 namespace Drupal\block_test\Plugin\Block;
 
+use Drupal\block\BlockBase;
+
 /**
  * Provides a block to test HTML.
  *