diff -u b/core/modules/system/tests/modules/router_test_directory/src/TestControllers.php b/core/modules/system/tests/modules/router_test_directory/src/TestControllers.php
--- b/core/modules/system/tests/modules/router_test_directory/src/TestControllers.php
+++ b/core/modules/system/tests/modules/router_test_directory/src/TestControllers.php
@@ -12,8 +12,7 @@
 use Drupal\user\UserInterface;
 use Symfony\Cmf\Component\Routing\RouteObjectInterface;
 use Symfony\Component\HttpFoundation\Response;
-use Zend\Diactoros\Response as PSRResponse;
-use Zend\Diactoros\Stream;
+use Zend\Diactoros\Response\HtmlResponse;
 
 
 /**
@@ -106,10 +105,7 @@
   }
 
   public function test23() {
-    $stream = new Stream('php://memory', 'wb+');
-    $stream->write('test23');
-    $response = new PSRResponse($stream);
-    return $response;
+    return new HtmlResponse('test23');
   }
 
   /**
diff -u b/core/tests/Drupal/Tests/Core/EventSubscriber/PsrResponseSubscriberTest.php b/core/tests/Drupal/Tests/Core/EventSubscriber/PsrResponseSubscriberTest.php
--- b/core/tests/Drupal/Tests/Core/EventSubscriber/PsrResponseSubscriberTest.php
+++ b/core/tests/Drupal/Tests/Core/EventSubscriber/PsrResponseSubscriberTest.php
@@ -47,7 +47,10 @@
    */
   public function testConvertsControllerResult() {
     $event = $this->createEventMock($this->getMock('Psr\Http\Message\ResponseInterface'));
-    $event->expects($this->once())->method('setResponse')->with($this->isInstanceOf('Symfony\Component\HttpFoundation\Response'));
+    $event
+      ->expects($this->once())
+      ->method('setResponse')
+      ->with($this->isInstanceOf('Symfony\Component\HttpFoundation\Response'));
     $this->psrResponseSubscriber->onKernelView($event);
   }
 
@@ -58,11 +61,15 @@
    * @covers ::onKernelView
    */
   public function testDoesNotConvertControllerResult() {
-    $event = $this->createEventMock(array());
-    $event->expects($this->never())->method('setResponse');
+    $event = $this->createEventMock([]);
+    $event
+      ->expects($this->never())
+      ->method('setResponse');
     $this->psrResponseSubscriber->onKernelView($event);
     $event = $this->createEventMock(NULL);
-    $event->expects($this->never())->method('setResponse');
+    $event
+      ->expects($this->never())
+      ->method('setResponse');
     $this->psrResponseSubscriber->onKernelView($event);
   }
 
@@ -76,11 +83,11 @@
    *    A mock object to test.
    */
   protected function createEventMock($controller_result) {
-    $event = $this->getMock('Symfony\Component\HttpKernel\Event\GetResponseForControllerResultEvent', array(), array(), '', NULL);
+    $event = $this->getMock('Symfony\Component\HttpKernel\Event\GetResponseForControllerResultEvent', [], [], '', NULL);
     $event
-        ->expects($this->any())
-        ->method('getControllerResult')
-        ->will($this->returnValue($controller_result));
+      ->expects($this->any())
+      ->method('getControllerResult')
+      ->will($this->returnValue($controller_result));
     return $event;
   }
 
@@ -91,11 +98,11 @@
    *    A mock object to test.
    */
   protected function createFactoryMock() {
-    $factory = $this->getMock('Symfony\Bridge\PsrHttpMessage\HttpFoundationFactoryInterface', array(), array(), '', NULL);
+    $factory = $this->getMock('Symfony\Bridge\PsrHttpMessage\HttpFoundationFactoryInterface', [], [], '', NULL);
     $factory
-        ->expects($this->any())
-        ->method('createResponse')
-        ->will($this->returnValue($this->getMock('Symfony\Component\HttpFoundation\Response')));
+      ->expects($this->any())
+      ->method('createResponse')
+      ->will($this->returnValue($this->getMock('Symfony\Component\HttpFoundation\Response')));
     return $factory;
   }