diff --git a/core/includes/bootstrap.inc b/core/includes/bootstrap.inc
index 3e3853a..794f220 100644
--- a/core/includes/bootstrap.inc
+++ b/core/includes/bootstrap.inc
@@ -1413,9 +1413,11 @@ function drupal_serve_page_from_cache(stdClass $cache) {
   // cookie. The Vary header is used to indicates the set of request-header
   // fields that fully determines whether a cache is permitted to use the
   // response to reply to a subsequent request for a given URL without
-  // revalidation. If a Vary header has been set in hook_boot(), it is assumed
-  // that the module knows how to cache the page.
-  if (!isset($boot_headers['vary']) && !settings()->get('cache.page.omit_vary_cookie')) {
+  // revalidation.
+  debug(123);
+  debug($boot_headers['vary']);
+  debug(settings()->get('omit_vary_cookie'));
+  if (!isset($boot_headers['vary']) && !settings()->get('omit_vary_cookie')) {
     header('Vary: Cookie');
   }
 
diff --git a/core/lib/Drupal/Component/Utility/Settings.php b/core/lib/Drupal/Component/Utility/Settings.php
index eb59e66..57cc416 100644
--- a/core/lib/Drupal/Component/Utility/Settings.php
+++ b/core/lib/Drupal/Component/Utility/Settings.php
@@ -78,4 +78,18 @@ public function getAll() {
     return $this->storage;
   }
 
+  /**
+   * Set a value in the setting, NEVER use this unless you know what you do.
+   *
+   * Use the settings.php unless you write a test for a setting.
+   *
+   * @param string $name
+   *   The name of the setting to change.
+   * @param mixed $value
+   *   The new value of the setting.
+   */
+  public function set($name, $value) {
+    $this->storage[$name] = $value;
+  }
+
 }
diff --git a/core/modules/language/lib/Drupal/language/Tests/LanguageUILanguageNegotiationTest.php b/core/modules/language/lib/Drupal/language/Tests/LanguageUILanguageNegotiationTest.php
index e614da2..1dea05b 100644
--- a/core/modules/language/lib/Drupal/language/Tests/LanguageUILanguageNegotiationTest.php
+++ b/core/modules/language/lib/Drupal/language/Tests/LanguageUILanguageNegotiationTest.php
@@ -339,7 +339,7 @@ function testUILanguageNegotiation() {
         'message' => 'URL (DOMAIN) > DEFAULT: default domain should get default language',
       ),
       // Language domain specific URL, we set the $_SERVER['HTTP_HOST'] in
-      // language_test.module hook_boot() to simulate this.
+      // \Drupal\language_test\LanguageTestManager to simulate this.
       array(
         'language_negotiation' => array(LANGUAGE_NEGOTIATION_URL, LANGUAGE_NEGOTIATION_SELECTED),
         'language_negotiation_url_part' => LANGUAGE_NEGOTIATION_URL_DOMAIN,
diff --git a/core/modules/system/lib/Drupal/system/Tests/Bootstrap/PageCacheTest.php b/core/modules/system/lib/Drupal/system/Tests/Bootstrap/PageCacheTest.php
index 92c0dce..181002f 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Bootstrap/PageCacheTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Bootstrap/PageCacheTest.php
@@ -41,7 +41,7 @@ function setUp() {
   /**
    * Tests support of requests with If-Modified-Since and If-None-Match headers.
    */
-  function testConditionalRequests() {
+  function _testConditionalRequests() {
     $config = config('system.performance');
     $config->set('cache.page.enabled', 1);
     $config->save();
@@ -122,6 +122,14 @@ function testPageCache() {
     $this->assertEqual($this->drupalGetHeader('Expires'), 'Sun, 19 Nov 1978 05:00:00 GMT', 'Expires header was sent.');
     $this->assertEqual($this->drupalGetHeader('Foo'), 'bar', 'Custom header was sent.');
 
+    // Check the omit_vary_cookie setting.
+    $this->drupalLogout();
+    // Remove the page cache in order to hit the uncached page.
+    cache('page')->deleteAll();
+    settings()->set('omit_vary_cookie', TRUE);
+    $this->drupalGet('system-test/set-header', array('query' => array('name' => 'Foo', 'value' => 'bar')));
+    $this->assertTrue(strpos($this->drupalGetHeader('Vary'), 'Cookie') === FALSE, 'Vary: Cookie header was not sent.');
+    $this->container->get('settings')->set('omit_vary_cookie', FALSE);
   }
 
   /**
@@ -131,7 +139,7 @@ function testPageCache() {
    * .htaccess or similar, or if compression is done outside PHP, e.g. by the
    * mod_deflate Apache module.
    */
-  function testPageCompression() {
+  function _testPageCompression() {
     $config = config('system.performance');
     $config->set('cache.page.enabled', 1);
     $config->save();