diff --git a/core/modules/block/block.module b/core/modules/block/block.module
index a95220d..8afc6de 100755
--- a/core/modules/block/block.module
+++ b/core/modules/block/block.module
@@ -603,14 +603,13 @@ function template_preprocess_block(&$variables) {
   // contains a hyphen, it will end up as an underscore after this conversion,
   // and your function names won't be recognized. So, we need to convert
   // hyphens to underscores in block deltas for the theme suggestions.
-  if (!empty($variables['block']->derivative)) {
-    list($plugin, $derivative) = explode(':', $variables['block']->id);
-    $variables['theme_hook_suggestions'][] = 'block__' . strtr($plugin, '-', '_');
-    $variables['theme_hook_suggestions'][] = 'block__' . strtr($plugin, '-', '_') . '__' . strtr($derivative, '-', '_');
-  }
-  else {
-    $plugin = $variables['block']->id;
-    $variables['theme_hook_suggestions'][] = 'block__' . strtr($plugin, '-', '_');
+
+  // We can safely explode on : because we know the Block plugin type manager
+  // enforces that delimiter for all derivatives.
+  $parts = explode(':', $variables['elements']['#block']->getPluginId());
+  $suggestion = 'block';
+  while ($part = array_shift($parts)) {
+    $variables['theme_hook_suggestions'][] = $suggestion .= '__' . strtr($part, '-', '_');
   }
   // Create a valid HTML ID and make sure it is unique.
   if (!empty($variables['block']->config_id)) {
diff --git a/core/modules/block/lib/Drupal/block/Tests/BlockTemplateSuggestionsUnitTest.php b/core/modules/block/lib/Drupal/block/Tests/BlockTemplateSuggestionsUnitTest.php
index 3b5b389..4078132 100644
--- a/core/modules/block/lib/Drupal/block/Tests/BlockTemplateSuggestionsUnitTest.php
+++ b/core/modules/block/lib/Drupal/block/Tests/BlockTemplateSuggestionsUnitTest.php
@@ -7,13 +7,20 @@
 
 namespace Drupal\block\Tests;
 
-use Drupal\simpletest\UnitTestBase;
-use stdClass;
+use Drupal\simpletest\WebTestBase;
 
 /**
  * Unit tests for template_preprocess_block().
  */
-class BlockTemplateSuggestionsUnitTest extends UnitTestBase {
+class BlockTemplateSuggestionsUnitTest extends WebTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  public static $modules = array('block');
+
   public static function getInfo() {
     return array(
       'name' => 'Block template suggestions',
@@ -26,31 +33,23 @@ public static function getInfo() {
    * Test if template_preprocess_block() handles the suggestions right.
    */
   function testBlockThemeHookSuggestions() {
-    // Define block delta with underscore to be preprocessed
-    $block1 = new stdClass();
-    $block1->module = 'block';
-    $block1->delta = 'underscore_test';
-    $block1->region = 'footer';
-    $variables1 = array();
-    $variables1['elements']['#block'] = $block1;
-    $variables1['elements']['#children'] = '';
-    template_preprocess_block($variables1);
-    $this->assertEqual($variables1['theme_hook_suggestions'], array('block__footer', 'block__block', 'block__block__underscore_test'), 'Found expected block suggestions for delta with underscore');
-
-    // Define block delta with hyphens to be preprocessed. Hyphens should be
-    // replaced with underscores.
-    $block2 = new stdClass();
-    $block2->module = 'block';
-    $block2->delta = 'hyphen-test';
-    $block2->region = 'footer';
-    $variables2 = array();
-    $variables2['elements']['#block'] = $block2;
-    $variables2['elements']['#children'] = '';
+    // Define a block with a derivative to be preprocessed, which includes both
+    // an underscore (not transformed) and a hyphen (transformed to underscore),
+    // and generates possibilities for each level of derivative.
+
+    $data = array(
+      'region' => 'footer',
+    );
+
+    // $block3 = block_load('system_powered_by_block', $data3);
+    $block = block_manager()->createInstance('system_menu_block:menu-admin', $data);
+    $variables = array();
+    $variables['elements']['#block'] = $block;
+    $variables['elements']['#children'] = '';
     // Test adding a class to the block content.
-    $variables2['content_attributes']['class'][] = 'test-class';
-    template_preprocess_block($variables2);
-    $this->assertEqual($variables2['theme_hook_suggestions'], array('block__footer', 'block__block', 'block__block__hyphen_test'), 'Hyphens (-) in block delta were replaced by underscore (_)');
-    // Test that the default class and added class are available.
-    $this->assertEqual($variables2['content_attributes']['class'], array('test-class', 'content'), 'Default .content class added to block content_attributes_array');
+    $variables['content_attributes']['class'][] = 'test-class';
+    template_preprocess_block($variables);
+    $this->assertEqual($variables['theme_hook_suggestions'], array('block__footer', 'block__system', 'block__system_menu_block', 'block__system_menu_block__menu_admin'));
+    $this->assertEqual($variables['content_attributes']['class'], array('test-class', 'content'), 'Default .content class added to block content_attributes_array');
   }
 }
diff --git a/core/modules/menu/lib/Drupal/menu/Plugin/Derivative/MenuBlock.php b/core/modules/menu/lib/Drupal/menu/Plugin/Derivative/MenuBlock.php
index 04a4c34..496a268 100644
--- a/core/modules/menu/lib/Drupal/menu/Plugin/Derivative/MenuBlock.php
+++ b/core/modules/menu/lib/Drupal/menu/Plugin/Derivative/MenuBlock.php
@@ -16,7 +16,6 @@ public function getDerivativeDefinitions(array $base_plugin_definition) {
     }
     foreach (menu_get_menus(FALSE) as $menu => $name) {
       $this->derivatives[$menu] = $base_plugin_definition;
-      $this->derivatives[$menu]['delta'] = $menu;
       $this->derivatives[$menu]['subject'] = t('Menu: @menu', array('@menu' => $name));
       $this->derivatives[$menu]['cache'] = DRUPAL_NO_CACHE;
     }