diff -u b/core/modules/toolbar/tests/src/FunctionalJavascript/ToolbarAdminMenuTest.php b/core/modules/toolbar/tests/src/FunctionalJavascript/ToolbarAdminMenuTest.php
--- b/core/modules/toolbar/tests/src/FunctionalJavascript/ToolbarAdminMenuTest.php
+++ b/core/modules/toolbar/tests/src/FunctionalJavascript/ToolbarAdminMenuTest.php
@@ -331,8 +331,8 @@
     // Request a new page to refresh the drupalSettings object.
     $subtrees_hash = $this->getSubtreesHash();
 
-    $ajax_result = $this->drupalGet('toolbar/subtrees/' . $subtrees_hash, ['query' => [MainContentViewSubscriber::WRAPPER_FORMAT => 'drupal_ajax']]);
-    $ajax_result = json_decode($ajax_result, TRUE);
+    $this->drupalGet('toolbar/subtrees/' . $subtrees_hash, ['query' => [MainContentViewSubscriber::WRAPPER_FORMAT => 'drupal_ajax']]);
+    $ajax_result = json_decode($this->xpath('//textarea')[0]->getValue(), TRUE);
     $this->assertResponse('200');
     $this->assertEqual($ajax_result[0]['command'], 'setToolbarSubtrees', 'Subtrees response uses the correct command.');
     $this->assertEqual(array_keys($ajax_result[0]['subtrees']), ['system-admin_content', 'system-admin_structure', 'system-themes_page', 'system-modules_list', 'system-admin_config', 'entity-user-collection', 'front'], 'Correct subtrees returned.');