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

core/lib/Drupal/Component/Utility/ToStringTrait.php

@@ -0,0 +1,48 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Component\Utility\ToStringTrait.
+ */
+
+namespace Drupal\Component\Utility;
+
+/**
+ * Wraps __toString in a trait to avoid some fatals.
+ */
+trait ToStringTrait {
+
+  /**
+   * Implements the magic __toString() method.
+   */
+  public function __toString() {
+    try {
+      return (string) $this->render();
+    }
+    catch (\Exception $e) {
+      // User errors in __toString() methods are considered fatal in the Drupal
+      // error handler.
+      trigger_error(get_class($e) . ' thrown while calling __toString on a ' . get_class($this) . ' object in ' . $e->getFile() . ' on line ' . $e->getLine() . ': ' . $e->getMessage(), E_USER_ERROR);
+      // In case we are using another error handler that did not fatal on the
+      // E_USER_ERROR, we terminate execution. However, for test purposes allow
+      // a return value.
+      return $this->_die();
+    }
+  }
+
+  /**
+   * For test purposes, wrap die() in an overridable method.
+   */
+  protected function _die() {
+    die();
+  }
+
+  /**
+   * Renders the object as a string.
+   *
+   * @return string|object
+   *   The rendered string or an object implementing __toString().
+   */
+  abstract public function render();
+
+}