.../Drupal/custom_block/Tests/CustomBlockFieldTest.php  |    7 ++++++-
 .../modules/custom_block_test/custom_block_test.module  |   15 +++++++++++++++
 2 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/core/modules/block/custom_block/lib/Drupal/custom_block/Tests/CustomBlockFieldTest.php b/core/modules/block/custom_block/lib/Drupal/custom_block/Tests/CustomBlockFieldTest.php
index 57f1e99..63f1daa 100644
--- a/core/modules/block/custom_block/lib/Drupal/custom_block/Tests/CustomBlockFieldTest.php
+++ b/core/modules/block/custom_block/lib/Drupal/custom_block/Tests/CustomBlockFieldTest.php
@@ -20,7 +20,7 @@ class CustomBlockFieldTest extends CustomBlockTestBase {
    *
    * @var array
    */
-  public static $modules = array('block', 'custom_block', 'link');
+  public static $modules = array('block', 'custom_block', 'custom_block_test', 'link');
 
   /**
    * The created field.
@@ -102,6 +102,10 @@ public function testBlockFields() {
     $this->drupalPostForm(NULL, $edit, t('Save'));
     $block = entity_load('custom_block', 1);
     $url = 'admin/structure/block/add/custom_block:' . $block->uuid() . '/' . \Drupal::config('system.theme')->get('default');
+    // Enable custom_block_test.module's hook_entity_view_alter(), which will
+    // set a data- attribute on the custom block entity. We want to ensure it
+    // shows up on the correct DOM node.
+    \Drupal::state()->set('custom_block_test_entity_view_alter_enabled', TRUE);
     // Place the block.
     $instance = array(
       'id' => drupal_strtolower($edit['info']),
@@ -113,6 +117,7 @@ public function testBlockFields() {
     $this->drupalGet('<front>');
     $this->assertLinkByHref('http://example.com');
     $this->assertText('Example.com');
+    $this->assertFieldByXPath('//div[@id="block-' . drupal_strtolower($edit['info']) . '" and @data-custom_block_test-entity-id="custom_block/1"]', NULL, 'data-custom_block_test-entity-id set on the correct DOM node.');
   }
 
 }
diff --git a/core/modules/block/custom_block/tests/modules/custom_block_test/custom_block_test.module b/core/modules/block/custom_block/tests/modules/custom_block_test/custom_block_test.module
index 7186098..c10e795 100644
--- a/core/modules/block/custom_block/tests/modules/custom_block_test/custom_block_test.module
+++ b/core/modules/block/custom_block/tests/modules/custom_block_test/custom_block_test.module
@@ -9,6 +9,8 @@
  */
 
 use Drupal\custom_block\Entity\CustomBlock;
+use Drupal\Core\Entity\EntityInterface;
+use Drupal\Core\Entity\Display\EntityViewDisplayInterface;
 
 /**
  * Implements hook_custom_block_view().
@@ -80,3 +82,16 @@ function custom_block_test_menu() {
   );
   return $items;
 }
+
+/**
+ * Implements hook_entity_view_alter().
+ */
+function custom_block_test_entity_view_alter(&$build, EntityInterface $entity, EntityViewDisplayInterface $display) {
+  // Allow tests to enable or disable this alter hook.
+  if (!\Drupal::state()->get('custom_block_test_entity_view_alter_enabled', FALSE)) {
+    return;
+  }
+
+  $build['#attributes']['data-custom_block_test-entity-id'] = $entity->entityType() . '/' . $entity->id();
+}
+