commit 712d677eaa18a3bf09b68299808ec714c2953e72
Author: Tamás Brückner <tamas.bruckner@integralvision.hu>
Date:   Fri May 12 12:21:48 2017 +0200

    HtmlEscapedText object to string.

diff --git a/core/lib/Drupal/Component/Render/HtmlEscapedText.php b/core/lib/Drupal/Component/Render/HtmlEscapedText.php
index b286f57..9d016e4 100644
--- a/core/lib/Drupal/Component/Render/HtmlEscapedText.php
+++ b/core/lib/Drupal/Component/Render/HtmlEscapedText.php
@@ -29,6 +29,13 @@ class HtmlEscapedText implements MarkupInterface, \Countable {
    *   The string to escape. This value will be cast to a string.
    */
   public function __construct($string) {
+    if (
+      is_object($string)
+      && method_exists($string, 'toString')
+    ) {
+      $string = $string->toString();
+    }
+
     $this->string = (string) $string;
   }