Update to drupal 8.0.0-rc1. For more information, see https://www.drupal.org/node/2582663

core/lib/Drupal/Component/Render/PlainTextOutput.php

@@ -0,0 +1,29 @@
+<?php
+/**
+ * @file
+ * Contains \Drupal\Component\Render\PlainTextOutput.
+ */
+
+namespace Drupal\Component\Render;
+
+use Drupal\Component\Utility\Html;
+
+/**
+ * Provides an output strategy for transforming HTML into simple plain text.
+ *
+ * Use this when rendering a given HTML string into a plain text string that
+ * does not need special formatting, such as a label or an email subject.
+ *
+ * Returns a string with HTML tags stripped and HTML entities decoded suitable
+ * for email or other non-HTML contexts.
+ */
+class PlainTextOutput implements OutputStrategyInterface {
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function renderFromHtml($string) {
+    return Html::decodeEntities(strip_tags((string) $string));
+  }
+
+}