Update to Drupal 8.0.0-beta15. For more information, see: https://www.drupal.org/node/2563023

core/modules/responsive_image/tests/modules/responsive_image_test_module/config/schema/responsive_image_test_module.schema.yml

@@ -0,0 +1,11 @@
+# Schema for the configuration files of the Responsive image test theme module.
+field.formatter.settings.responsive_image_test:
+  type: mapping
+  label: 'Responsive image list format settings'
+  mapping:
+    responsive_image_style:
+      type: string
+      label: 'Responsive image style'
+    image_link:
+      type: string
+      label: 'Link image to'

core/modules/responsive_image/tests/modules/responsive_image_test_module/src/Plugin/Field/FieldFormatter/ResponsiveImageTestFormatter.php

@@ -0,0 +1,39 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\responsive_image_test_module\Plugin\Field\FieldFormatter\ResponsiveImageTestFormatter.
+ */
+
+namespace Drupal\responsive_image_test_module\Plugin\Field\FieldFormatter;
+
+use Drupal\responsive_image\Plugin\Field\FieldFormatter\ResponsiveImageFormatter;
+use Drupal\Core\Field\FieldItemListInterface;
+
+/**
+ * Plugin to test responsive image formatter.
+ *
+ * @FieldFormatter(
+ *   id = "responsive_image_test",
+ *   label = @Translation("Responsive image test"),
+ *   field_types = {
+ *     "image",
+ *   }
+ * )
+ */
+class ResponsiveImageTestFormatter extends ResponsiveImageFormatter {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function viewElements(FieldItemListInterface $items) {
+    $elements = parent::viewElements($items);
+    // Unset #item_attributes to test that the theme function can handle that.
+    foreach ($elements as &$element) {
+      if (isset($element['#item_attributes'])) {
+        unset($element['#item_attributes']);
+      }
+    }
+    return $elements;
+  }
+}