reverted:
--- b/core/modules/system/system.module
+++ a/core/modules/system/system.module
@@ -104,9 +104,6 @@
 
     case 'system.themes_page':
       $output = '<p>' . t('Set and configure the default theme for your website.  Alternative <a href="!themes">themes</a> are available.', array('!themes' => 'https://www.drupal.org/project/themes')) . '</p>';
-      if (\Drupal::moduleHandler()->moduleExists('block')) {
-        $output .= '<p>' . t('Do you want to <a href="@blocks">insert a block</a> into the layout?', array('@blocks' => \Drupal::url('block.admin_display'))) . '</p>';
-      }
       return $output;
 
     case 'system.theme_settings_theme':
only in patch2:
unchanged:
--- a/core/modules/block/block.module
+++ b/core/modules/block/block.module
@@ -39,13 +39,16 @@ function block_help($route_name, RouteMatchInterface $route_match) {
       $output .= '<dd>' . t('You can add custom blocks, if the <em>Custom Block</em> module is installed. For more information, see the <a href="!blockcontent-help">Custom Block help page</a>.', array('!blockcontent-help' => $block_content)) . '</dd>';
       $output .= '</dl>';
       return $output;
-  }
-  if ($route_name == 'block.admin_display' || $route_name == 'block.admin_display_theme') {
-    $demo_theme = $route_match->getParameter('theme') ?: \Drupal::config('system.theme')->get('default');
-    $themes = \Drupal::service('theme_handler')->listInfo();
-    $output = '<p>' . t('This page provides a drag-and-drop interface for adding a block to a region, and for controlling the order of blocks within regions. To add a block to a region, or to configure its specific title and visibility settings, click the block title under <em>Place blocks</em>. Since not all themes implement the same regions, or display regions in the same way, blocks are positioned on a per-theme basis. Remember that your changes will not be saved until you click the <em>Save blocks</em> button at the bottom of the page.') . '</p>';
-    $output .= '<p>' . \Drupal::l(t('Demonstrate block regions (@theme)', array('@theme' => $themes[$demo_theme]->info['name'])), new Url('block.admin_demo', array('theme' => $demo_theme))) . '</p>';
-    return $output;
+    case 'system.themes_page':
+      $output = '<p>' . t('Do you want to <a href="@blocks">insert a block</a> into the layout?', array('@blocks' => \Drupal::url('block.admin_display'))) . '</p>';
+      return $output;
+    case 'block.admin_display':
+    case 'block.admin_display_theme':
+      $demo_theme = $route_match->getParameter('theme') ?: \Drupal::config('system.theme')->get('default');
+      $themes = \Drupal::service('theme_handler')->listInfo();
+      $output = '<p>' . t('This page provides a drag-and-drop interface for adding a block to a region, and for controlling the order of blocks within regions. To add a block to a region, or to configure its specific title and visibility settings, click the block title under <em>Place blocks</em>. Since not all themes implement the same regions, or display regions in the same way, blocks are positioned on a per-theme basis. Remember that your changes will not be saved until you click the <em>Save blocks</em> button at the bottom of the page.') . '</p>';
+      $output .= '<p>' . \Drupal::l(t('Demonstrate block regions (@theme)', array('@theme' => $themes[$demo_theme]->info['name'])), new Url('block.admin_demo', array('theme' => $demo_theme))) . '</p>';
+      return $output;
   }
 }