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

core/modules/responsive_image/src/Element/ResponsiveImage.php

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+<?php
+/**
+ * @file
+ * Contains \Drupal\responsive_image\Element\ResponsiveImage.
+ */
+
+namespace Drupal\responsive_image\Element;
+
+use Drupal\Core\Render\Element\RenderElement;
+
+/**
+ * Provides a responsive image element.
+ *
+ * @RenderElement("responsive_image")
+ */
+class ResponsiveImage extends RenderElement {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getInfo() {
+    return [
+      '#theme' => 'responsive_image',
+      '#attached' => [
+        'library' => ['core/picturefill'],
+      ],
+    ];
+  }
+
+}