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

core/modules/aggregator/templates/aggregator-feed.html.twig

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+{#
+/**
+ * @file
+ * Default theme implementation to present an aggregator feed.
+ *
+ * The contents are rendered above feed listings when browsing source feeds.
+ * For example, "example.com/aggregator/sources/1".
+ *
+ * Available variables:
+ * - title: Title of the feed item.
+ * - content: All field items. Use {{ content }} to print them all,
+ *   or print a subset such as {{ content.field_example }}. Use
+ *   {{ content|without('field_example') }} to temporarily suppress the printing
+ *   of a given element.
+ *
+ * @see template_preprocess_aggregator_feed()
+ *
+ * @ingroup themeable
+ */
+#}
+{{ title_prefix }}
+{% if not full %}
+  <h2{{ title_attributes }}>{{ title }}</h2>
+{% endif %}
+{{ title_suffix }}
+
+{{ content }}

core/modules/aggregator/templates/aggregator-item.html.twig

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+{#
+/**
+ * @file
+ * Default theme implementation to present a feed item in an aggregator page.
+ *
+ * Available variables:
+ * - url: URL to the originating feed item.
+ * - title: Title of the feed item.
+ * - content: All field items. Use {{ content }} to print them all,
+ *   or print a subset such as {{ content.field_example }}. Use
+ *   {{ content|without('field_example') }} to temporarily suppress the printing
+ *   of a given element.
+ *
+ * @see template_preprocess_aggregator_item()
+ *
+ * @ingroup themeable
+ */
+#}
+<article{{ attributes }}>
+  {{ title_prefix }}
+  <h3>
+    <a href="{{ url }}">{{ title }}</a>
+  </h3>
+  {{ title_suffix }}
+  {{ content }}
+</article>