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

core/modules/image/templates/image-formatter.html.twig

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+{#
+/**
+ * @file
+ * Default theme implementation to display a formatted image field.
+ *
+ * Available variables:
+ * - image: A collection of image data.
+ * - image_style: An optional image style.
+ * - url: An optional URL the image can be linked to.
+ *
+ * @see template_preprocess_image_formatter()
+ *
+ * @ingroup themeable
+ */
+#}
+{% if url %}
+  <a href="{{ url }}">{{ image }}</a>
+{% else %}
+  {{ image }}
+{% endif %}