Drupal 8.0.0 beta 12. More info: https://www.drupal.org/node/2514176

core/modules/block_content/block_content.pages.inc

@@ -0,0 +1,38 @@
+<?php
+
+/**
+ * @file
+ * Provides page callbacks for custom blocks.
+ */
+
+use Drupal\Component\Utility\Xss;
+use Drupal\Core\Url;
+use Drupal\block_content\Entity\BlockContentType;
+use Drupal\block_content\Entity\BlockContent;
+use Symfony\Component\HttpFoundation\RedirectResponse;
+
+/**
+ * Prepares variables for a custom block type creation list templates.
+ *
+ * Default template: block-content-add-list.html.twig.
+ *
+ * @param array $variables
+ *   An associative array containing:
+ *   - content: An array of block types.
+ *
+ * @see block_content_add_page()
+ */
+function template_preprocess_block_content_add_list(&$variables) {
+  $variables['types'] = array();
+  $query = \Drupal::request()->query->all();
+  foreach ($variables['content'] as $type) {
+    $variables['types'][$type->id()] = array(
+      'link' => \Drupal::l($type->label(), new Url('block_content.add_form', array('block_content_type' => $type->id()), array('query' => $query))),
+      'description' => Xss::filterAdmin($type->getDescription()),
+      'title' => $type->label(),
+      'localized_options' => array(
+        'query' => $query,
+      ),
+    );
+  }
+}