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

core/modules/system/templates/admin-page.html.twig

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+{#
+/**
+ * @file
+ * Default theme implementation for an administrative page.
+ *
+ * Available variables:
+ * - system_compact_link: Themed link to toggle compact view.
+ * - containers: An list of administrative blocks keyed by position: left or
+ *   right. Contains:
+ *   - blocks: A list of blocks within a container.
+ *
+ * @see template_preprocess_admin_page()
+ *
+ * @ingroup themeable
+ */
+#}
+<div class="clearfix">
+  {{ system_compact_link }}
+  {% for container in containers %}
+    <div class="layout-column half">
+      {% for block in container.blocks %}
+        {{ block }}
+      {% endfor %}
+    </div>
+  {% endfor %}
+</div>