Update to Drupal 8.1.2. For more information, see https://www.drupal.org/project/drupal/releases/8.1.2

vendor/behat/mink/src/Selector/CssSelector.php

@@ -11,6 +11,7 @@
 namespace Behat\Mink\Selector;
 
 use Symfony\Component\CssSelector\CssSelector as CSS;
+use Symfony\Component\CssSelector\CssSelectorConverter;
 
 /**
  * CSS selector engine. Transforms CSS to XPath.
@@ -32,6 +33,14 @@ class CssSelector implements SelectorInterface
             throw new \InvalidArgumentException('The CssSelector expects to get a string as locator');
         }
 
+        // Symfony 2.8+ API
+        if (class_exists('Symfony\Component\CssSelector\CssSelectorConverter')) {
+            $converter = new CssSelectorConverter();
+
+            return $converter->toXPath($locator);
+        }
+
+        // old static API for Symfony 2.7 and older
         return CSS::toXPath($locator);
     }
 }