Update to Drupal 8.1.0. For more information, see https://www.drupal.org/drupal-8.1.0-release-notes

vendor/symfony/dependency-injection/Reference.php

@@ -25,6 +25,8 @@ class Reference
     /**
      * Constructor.
      *
+     * Note: The $strict parameter is deprecated since version 2.8 and will be removed in 3.0.
+     *
      * @param string $id              The service identifier
      * @param int    $invalidBehavior The behavior when the service does not exist
      * @param bool   $strict          Sets how this reference is validated
@@ -62,9 +64,15 @@ class Reference
      * Returns true when this Reference is strict.
      *
      * @return bool
+     *
+     * @deprecated since version 2.8, to be removed in 3.0.
      */
-    public function isStrict()
+    public function isStrict($triggerDeprecationError = true)
     {
+        if ($triggerDeprecationError) {
+            @trigger_error('The '.__METHOD__.' method is deprecated since version 2.8 and will be removed in 3.0.', E_USER_DEPRECATED);
+        }
+
         return $this->strict;
     }
 }