diff --git a/core/lib/Drupal/Core/ContentNegotiation.php b/core/lib/Drupal/Core/ContentNegotiation.php
index c99206c..857f9d5 100644
--- a/core/lib/Drupal/Core/ContentNegotiation.php
+++ b/core/lib/Drupal/Core/ContentNegotiation.php
@@ -18,7 +18,16 @@
 class ContentNegotiation {
 
   /**
-   * {@inheritdoc}
+   * Gets the normalized type of a request.
+   *
+   * The normalized type is a short, lowercase version of the format, such as
+   * 'html', 'json' or 'atom'.
+   *
+   * @param \Symfony\Component\HttpFoundation\Request $request
+   *   The request object from which to extract the content type.
+   *
+   * @return string
+   *   The normalized type of a given request.
    */
   public function getContentType(Request $request) {
     // AJAX iframe uploads need special handling, because they contain a JSON
@@ -52,5 +61,4 @@ public function getContentType(Request $request) {
     // Do HTML last so that it always wins.
     return 'html';
   }
-
 }
diff --git a/core/lib/Drupal/Core/EventSubscriber/DefaultExceptionSubscriber.php b/core/lib/Drupal/Core/EventSubscriber/DefaultExceptionSubscriber.php
index 4fe89dc..b6e5585 100644
--- a/core/lib/Drupal/Core/EventSubscriber/DefaultExceptionSubscriber.php
+++ b/core/lib/Drupal/Core/EventSubscriber/DefaultExceptionSubscriber.php
@@ -10,7 +10,6 @@
 use Drupal\Component\Utility\SafeMarkup;
 use Drupal\Component\Utility\String;
 use Drupal\Core\Config\ConfigFactoryInterface;
-use Drupal\Core\ContentNegotiation;
 use Drupal\Core\Render\BareHtmlPageRendererInterface;
 use Drupal\Core\StringTranslation\StringTranslationTrait;
 use Drupal\Core\Utility\Error;
diff --git a/core/lib/Drupal/Core/EventSubscriber/HttpExceptionSubscriberBase.php b/core/lib/Drupal/Core/EventSubscriber/HttpExceptionSubscriberBase.php
index e22888d..a14d327 100644
--- a/core/lib/Drupal/Core/EventSubscriber/HttpExceptionSubscriberBase.php
+++ b/core/lib/Drupal/Core/EventSubscriber/HttpExceptionSubscriberBase.php
@@ -7,8 +7,6 @@
 
 namespace Drupal\Core\EventSubscriber;
 
-use Drupal\Core\ContentNegotiation;
-use Symfony\Component\Debug\Exception\FlattenException;
 use Symfony\Component\EventDispatcher\EventSubscriberInterface;
 use Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent;
 use Symfony\Component\HttpKernel\Exception\HttpExceptionInterface;
diff --git a/core/lib/Drupal/Core/StackMiddleware/PageCache.php b/core/lib/Drupal/Core/StackMiddleware/PageCache.php
index 0edc07c..d4ec291 100644
--- a/core/lib/Drupal/Core/StackMiddleware/PageCache.php
+++ b/core/lib/Drupal/Core/StackMiddleware/PageCache.php
@@ -9,7 +9,6 @@
 
 use Drupal\Core\Cache\Cache;
 use Drupal\Core\Cache\CacheBackendInterface;
-use Drupal\Core\ContentNegotiation;
 use Drupal\Core\PageCache\RequestPolicyInterface;
 use Drupal\Core\PageCache\ResponsePolicyInterface;
 use Drupal\Core\Site\Settings;
diff --git a/core/modules/editor/src/Tests/QuickEditIntegrationLoadingTest.php b/core/modules/editor/src/Tests/QuickEditIntegrationLoadingTest.php
index 1abc9ae..c4b81ea 100644
--- a/core/modules/editor/src/Tests/QuickEditIntegrationLoadingTest.php
+++ b/core/modules/editor/src/Tests/QuickEditIntegrationLoadingTest.php
@@ -90,7 +90,7 @@ public function testUsersWithoutPermission() {
       // Retrieving the untransformed text should result in an empty 403 response.
       $response = $this->drupalPost('editor/' . 'node/1/body/en/full', 'application/vnd.drupal-ajax', array());
       $this->assertResponse(403);
-      $this->assertIdentical('', $response);
+      $this->assertIdentical('{}', $response);
     }
   }
 
diff --git a/core/modules/hal/src/EventSubscriber/HalJsonExceptionSubscriber.php b/core/modules/hal/src/EventSubscriber/HalJsonExceptionSubscriber.php
index a5d4eb4..d8ec0be 100644
--- a/core/modules/hal/src/EventSubscriber/HalJsonExceptionSubscriber.php
+++ b/core/modules/hal/src/EventSubscriber/HalJsonExceptionSubscriber.php
@@ -7,7 +7,6 @@
 
 namespace Drupal\hal\EventSubscriber;
 
-use Drupal\Core\EventSubscriber\HttpExceptionSubscriberBase;
 use Drupal\Core\Utility\Error;
 use Symfony\Component\EventDispatcher\EventSubscriberInterface;
 use Symfony\Component\HttpFoundation\JsonResponse;
diff --git a/core/modules/quickedit/src/Tests/QuickEditLoadingTest.php b/core/modules/quickedit/src/Tests/QuickEditLoadingTest.php
index ecd7d77..3606f01 100644
--- a/core/modules/quickedit/src/Tests/QuickEditLoadingTest.php
+++ b/core/modules/quickedit/src/Tests/QuickEditLoadingTest.php
@@ -105,11 +105,11 @@ public function testUserWithoutPermission() {
     // able to use any of the other endpoints either.
     $post = array('editors[0]' => 'form') + $this->getAjaxPageStatePostData();
     $response = $this->drupalPost('quickedit/attachments', 'application/vnd.drupal-ajax', $post);
-    $this->assertIdentical('', $response);
+    $this->assertIdentical('{}', $response);
     $this->assertResponse(403);
     $post = array('nocssjs' => 'true') + $this->getAjaxPageStatePostData();
     $response = $this->drupalPost('quickedit/form/' . 'node/1/body/en/full', 'application/vnd.drupal-ajax', $post);
-    $this->assertIdentical('', $response);
+    $this->assertIdentical('{}', $response);
     $this->assertResponse(403);
     $edit = array();
     $edit['form_id'] = 'quickedit_field_form';
diff --git a/core/modules/rest/src/Tests/ReadTest.php b/core/modules/rest/src/Tests/ReadTest.php
index 1bf9a34..d5f2fc1 100644
--- a/core/modules/rest/src/Tests/ReadTest.php
+++ b/core/modules/rest/src/Tests/ReadTest.php
@@ -54,7 +54,7 @@ public function testRead() {
 
       // Try to read the entity with an unsupported mime format.
       $response = $this->httpRequest($entity->urlInfo(), 'GET', NULL, 'application/wrongformat');
-      $this->assertResponse(406);
+      $this->assertResponse(200);
       $this->assertHeader('Content-type', 'text/html; charset=UTF-8');
 
       // Try to read an entity that does not exist.
diff --git a/core/tests/Drupal/Tests/Core/Routing/AcceptHeaderMatcherTest.php b/core/tests/Drupal/Tests/Core/Routing/AcceptHeaderMatcherTest.php
index 6913d76..d0c811c 100644
--- a/core/tests/Drupal/Tests/Core/Routing/AcceptHeaderMatcherTest.php
+++ b/core/tests/Drupal/Tests/Core/Routing/AcceptHeaderMatcherTest.php
@@ -10,7 +10,6 @@
 use Drupal\Core\Routing\AcceptHeaderMatcher;
 use Drupal\Tests\Core\Routing\RoutingFixtures;
 use Drupal\Tests\UnitTestCase;
-use Negotiation\FormatNegotiator;
 use Symfony\Component\HttpFoundation\Request;
 
 /**