Split up system/base library.

Closes #2880237

core/lib/Drupal/Core/Render/Element/Fieldgroup.php

@@ -22,6 +22,7 @@ class Fieldgroup extends Fieldset {
   public function getInfo() {
     return [
       '#attributes' => ['class' => ['fieldgroup']],
+      '#attach' => ['library' => ['core/drupal.fieldgroup']],
     ] + parent::getInfo();
   }