Update to Drupal 8.0.0-rc3. For more information, see https://www.drupal.org/node/2608078

vendor/jcalderonzumba/mink-phantomjs-driver/src/Resources/Script/is_selected.js.twig

@@ -0,0 +1,16 @@
+{% autoescape 'js' %}
+(function (xpath) {
+  function getElement(xpath) {
+    var polterAgent = window.__poltergeist;
+    var ids = polterAgent.find("xpath", xpath, document);
+    var polterNode = polterAgent.get(ids[0]);
+    return polterNode.element;
+  }
+
+  var node = getElement(xpath);
+  if(typeof node.selected == "undefined"){
+    return null;
+  }
+  return node.selected;
+}('{{xpath}}'));
+{% endautoescape %}