reverted:
--- b/core/modules/system/tests/modules/test_page_test/src/Controller/Test.php
+++ a/core/modules/system/tests/modules/test_page_test/src/Controller/Test.php
@@ -1,7 +1,6 @@
 <?php
 
 namespace Drupal\test_page_test\Controller;
-use Symfony\Component\HttpFoundation\JsonResponse;
 use Symfony\Component\HttpKernel\Exception\HttpException;
 
 /**
@@ -109,14 +108,4 @@
     return ['#markup' => '<a href="http://example.com">foo|bar|baz</a>'];
   }
 
-  /**
-   * Renders some simple json.
-   *
-   * @return \Symfony\Component\HttpFoundation\JsonResponse
-   *   A json response.
-   */
-  public function renderJson() {
-    return new JsonResponse(['some' => 'data']);
-  }
-
 }
reverted:
--- b/core/modules/system/tests/modules/test_page_test/test_page_test.routing.yml
+++ a/core/modules/system/tests/modules/test_page_test/test_page_test.routing.yml
@@ -82,11 +82,3 @@
     _form: '\Drupal\test_page_test\Form\TestForm'
   requirements:
     _access: 'TRUE'
-
-test_page_test.json:
-  path: '/test-json'
-  defaults:
-    _title: 'Simple json output'
-    _controller: '\Drupal\test_page_test\Controller\Test::renderJson'
-  requirements:
-    _access: 'TRUE'
diff -u b/core/tests/Drupal/FunctionalTests/BrowserTestBaseTest.php b/core/tests/Drupal/FunctionalTests/BrowserTestBaseTest.php
--- b/core/tests/Drupal/FunctionalTests/BrowserTestBaseTest.php
+++ b/core/tests/Drupal/FunctionalTests/BrowserTestBaseTest.php
@@ -146,6 +146,9 @@
     $sanitized = Html::escape($dangerous);
     $this->assertNoText($dangerous);
     $this->assertText($sanitized);
+
+    // Test getRawContent().
+    $this->assertSame($this->getSession()->getPage()->getContent(), $this->getRawContent());
   }
 
   /**
@@ -234,10 +237,2 @@
 
-  /**
-   * Tests the getRawContent() method.
-   */
-  public function testRawContent() {
-    $this->drupalGet('test-json');
-    $this->assertEquals('{"some":"data"}', $this->getRawContent());
-  }
-
 }