diff --git a/core/includes/ajax.inc b/core/includes/ajax.inc
index 45bb706..7395b58 100644
--- a/core/includes/ajax.inc
+++ b/core/includes/ajax.inc
@@ -326,7 +326,7 @@ function ajax_get_form() {
     // This is likely a hacking attempt as it never happens under normal
     // circumstances, so we just do nothing.
     watchdog('ajax', 'Invalid form POST data.', array(), WATCHDOG_WARNING);
-    drupal_exit();
+    return new Response('Forbidden', 403);
   }
 
   // Since some of the submit handlers are run, redirects need to be disabled.
diff --git a/core/includes/bootstrap.inc b/core/includes/bootstrap.inc
index eac2f97..b3e2d32 100644
--- a/core/includes/bootstrap.inc
+++ b/core/includes/bootstrap.inc
@@ -617,8 +617,7 @@ function drupal_environment_initialize() {
     $_SERVER['HTTP_HOST'] = strtolower($_SERVER['HTTP_HOST']);
     if (!drupal_valid_http_host($_SERVER['HTTP_HOST'])) {
       // HTTP_HOST is invalid, e.g. if containing slashes it may be an attack.
-      header($_SERVER['SERVER_PROTOCOL'] . ' 400 Bad Request');
-      exit;
+      return new Response('Bad Request', 400);
     }
   }
   else {
@@ -639,9 +638,7 @@ function drupal_environment_initialize() {
 
   // Deny execution with enabled "magic quotes" (both GPC and runtime).
   if (get_magic_quotes_gpc() || get_magic_quotes_runtime()) {
-    header($_SERVER['SERVER_PROTOCOL'] . ' 500 Internal Server Error');
-    print "PHP's 'magic_quotes_gpc' and 'magic_quotes_runtime' settings are not supported and must be disabled.";
-    exit;
+    return new Response("PHP's 'magic_quotes_gpc' and 'magic_quotes_runtime' settings are not supported and must be disabled.", 500);
   }
 
   // Use session cookies, not transparent sessions that puts the session id in
@@ -2302,7 +2299,7 @@ function _drupal_bootstrap_page_cache() {
         bootstrap_invoke_all('exit');
       }
       // We are done.
-      exit;
+      return;
     }
     else {
       header('X-Drupal-Cache: MISS');
diff --git a/core/includes/errors.inc b/core/includes/errors.inc
index 92ba6bc..7c9b03d 100644
--- a/core/includes/errors.inc
+++ b/core/includes/errors.inc
@@ -215,7 +215,7 @@ function _drupal_log_error($error, $fatal = FALSE) {
       // When called from CLI, simply output a plain text message.
       // Should not translate the string to avoid errors producing more errors.
       print html_entity_decode(strip_tags(format_string('%type: !message in %function (line %line of %file).', $error))). "\n";
-      exit;
+      return;
     }
   }
 
@@ -226,7 +226,7 @@ function _drupal_log_error($error, $fatal = FALSE) {
         // Should not translate the string to avoid errors producing more errors.
         print format_string('%type: !message in %function (line %line of %file).', $error);
       }
-      exit;
+      return;
     }
   }
   else {
diff --git a/core/modules/system/tests/http.php b/core/modules/system/tests/http.php
index 297e3c7..b8400fd 100644
--- a/core/modules/system/tests/http.php
+++ b/core/modules/system/tests/http.php
@@ -24,18 +24,17 @@
 define('DRUPAL_ROOT', getcwd());
 require_once DRUPAL_ROOT . '/core/includes/bootstrap.inc';
 
-// Make sure this file can only be used by simpletest.
-drupal_bootstrap(DRUPAL_BOOTSTRAP_CONFIGURATION);
-if (!drupal_valid_test_ua()) {
-  header($_SERVER['SERVER_PROTOCOL'] . ' 403 Forbidden');
-  exit;
-}
-
 // Continue with normal request handling.
 $request = Request::createFromGlobals();
 
 drupal_bootstrap(DRUPAL_BOOTSTRAP_CODE);
-
 $kernel = new DrupalKernel('prod', FALSE);
-$response = $kernel->handle($request)->prepare($request)->send();
+// Make sure this file can only be used by simpletest.
+if (!drupal_valid_test_ua()) {
+  $response = new Response('Forbidden', 403);
+}
+else {
+  $response = $kernel->handle($request)->prepare($request)->send();
+}
 $kernel->terminate($request, $response);
+
diff --git a/core/modules/system/tests/https.php b/core/modules/system/tests/https.php
index 8e09a5d..9b7066a 100644
--- a/core/modules/system/tests/https.php
+++ b/core/modules/system/tests/https.php
@@ -23,18 +23,18 @@
 define('DRUPAL_ROOT', getcwd());
 require_once DRUPAL_ROOT . '/core/includes/bootstrap.inc';
 
-// Make sure this file can only be used by simpletest.
-drupal_bootstrap(DRUPAL_BOOTSTRAP_CONFIGURATION);
-if (!drupal_valid_test_ua()) {
-  header($_SERVER['SERVER_PROTOCOL'] . ' 403 Forbidden');
-  exit;
-}
 
 // Continue with normal request handling.
 $request = Request::createFromGlobals();
 
 drupal_bootstrap(DRUPAL_BOOTSTRAP_CODE);
-
 $kernel = new DrupalKernel('prod', FALSE);
-$response = $kernel->handle($request)->prepare($request)->send();
+// Make sure this file can only be used by simpletest.
+if (!drupal_valid_test_ua()) {
+  $response = new Response('Forbidden', 403);
+}
+else {
+  $response = $kernel->handle($request)->prepare($request)->send();
+}
 $kernel->terminate($request, $response);
+
diff --git a/core/modules/system/tests/modules/form_test/form_test.module b/core/modules/system/tests/modules/form_test/form_test.module
index 03b7cb8..f7ea120 100644
--- a/core/modules/system/tests/modules/form_test/form_test.module
+++ b/core/modules/system/tests/modules/form_test/form_test.module
@@ -6,6 +6,7 @@
  */
 
 use Drupal\form_test\Callbacks;
+use \Symfony\Component\HttpFoundation\JsonResponse;
 
 /**
  * Implements hook_menu().
@@ -328,10 +329,7 @@ function form_test_permission() {
  * Form submit handler to return form values as JSON.
  */
 function _form_test_submit_values_json($form, &$form_state) {
-  // This won't have a proper JSON header, but Drupal doesn't check for that
-  // anyway so this is fine until it's replaced with a JsonResponse.
-  print drupal_json_encode($form_state['values']);
-  drupal_exit();
+  return new JsonResponse($form_state['values']);
 }
 
 /**
@@ -1056,10 +1054,9 @@ function form_test_form_state_values_clean_form($form, &$form_state) {
  */
 function form_test_form_state_values_clean_form_submit($form, &$form_state) {
   form_state_values_clean($form_state);
-  // This won't have a proper JSON header, but Drupal doesn't check for that
-  // anyway so this is fine until it's replaced with a JsonResponse.
-  print drupal_json_encode($form_state['values']);
-  exit;
+  $response = new JsonResponse($form_state['values']);
+  $response->send();
+  return FALSE;
 }
 
 /**
@@ -1086,7 +1083,7 @@ function form_test_form_state_values_clean_advanced_form($form, &$form_state) {
 function form_test_form_state_values_clean_advanced_form_submit($form, &$form_state) {
   form_state_values_clean($form_state);
   print t('You WIN!');
-  exit;
+  return FALSE;
 }
 
 /**
diff --git a/core/modules/system/tests/modules/session_test/session_test.module b/core/modules/system/tests/modules/session_test/session_test.module
index 3b378ad..294fa9b 100644
--- a/core/modules/system/tests/modules/session_test/session_test.module
+++ b/core/modules/system/tests/modules/session_test/session_test.module
@@ -157,7 +157,7 @@ function session_test_user_login($edit = array(), $user = NULL) {
   if ($user->name == 'session_test_user') {
     // Exit so we can verify that the session was regenerated
     // before hook_user() was called.
-    exit;
+    return new Response('OK', 200);
   }
 }
 
diff --git a/core/modules/system/tests/modules/system_test/system_test.module b/core/modules/system/tests/modules/system_test/system_test.module
index 4949aec..73edfe0 100644
--- a/core/modules/system/tests/modules/system_test/system_test.module
+++ b/core/modules/system/tests/modules/system_test/system_test.module
@@ -123,8 +123,7 @@ function system_test_redirect($code) {
   $code = (int) $code;
   if ($code != 200) {
     // Header names are case-insensitive.
-    header("locaTION: " . url('system-test/redirect/200', array('absolute' => TRUE)), TRUE, $code);
-    exit;
+    return new RedirectResponse("locaTION: " . url('system-test/redirect/200', array('absolute' => TRUE)), $code);
   }
   return '';
 }
@@ -147,8 +146,8 @@ function system_test_redirect($code) {
 function system_test_multiple_redirects($count) {
   $count = (int) $count;
   if ($count > 0) {
-    header("location: " . url('system-test/multiple-redirects/' . --$count, array('absolute' => TRUE)), TRUE, 301);
-    exit;
+    // Header names are case-insensitive.
+    return new RedirectResponse("location: " . url('system-test/multiple-redirects/' . --$count, array('absolute' => TRUE)), 301);
   }
   return '';
 }
@@ -159,18 +158,15 @@ function system_test_set_header() {
 }
 
 function system_test_redirect_noscheme() {
-  header("Location: localhost/path", TRUE, 301);
-  exit;
+  return new RedirectResponse("Location: localhost/path", 301);
 }
 
 function system_test_redirect_noparse() {
-  header("Location: http:///path", TRUE, 301);
-  exit;
+  return new RedirectResponse("Location: http:///path", 301);
 }
 
 function system_test_redirect_invalid_scheme() {
-  header("Location: ftp://localhost/path", TRUE, 301);
-  exit;
+  return new RedirectResponse("Location: ftp://localhost/path", 301);
 }
 
 /**
diff --git a/core/modules/system/tests/modules/url_alter_test/url_alter_test.module b/core/modules/system/tests/modules/url_alter_test/url_alter_test.module
index a5ca13d..4637454 100644
--- a/core/modules/system/tests/modules/url_alter_test/url_alter_test.module
+++ b/core/modules/system/tests/modules/url_alter_test/url_alter_test.module
@@ -22,8 +22,7 @@ function url_alter_test_menu() {
  * Menu callback.
  */
 function url_alter_test_foo() {
-  print 'current_path=' . current_path() . ' request_path=' . request_path();
-  exit;
+  return new Response('current_path=' . current_path() . ' request_path=' . request_path(), 200);
 }
 
 /**
diff --git a/core/modules/taxonomy/taxonomy.pages.inc b/core/modules/taxonomy/taxonomy.pages.inc
index 042263c..0baa1c9 100644
--- a/core/modules/taxonomy/taxonomy.pages.inc
+++ b/core/modules/taxonomy/taxonomy.pages.inc
@@ -122,8 +122,9 @@ function taxonomy_autocomplete($field_name, $tags_typed = '') {
   if (!($field = field_info_field($field_name)) || $field['type'] !== 'taxonomy_term_reference') {
     // Error string. The JavaScript handler will realize this is not JSON and
     // will display it as debugging information.
-    print t('Taxonomy field @field_name not found.', array('@field_name' => $field_name));
-    exit;
+    $response = new JsonResponse();
+    $response->setContent(t('Taxonomy field @field_name not found.', array('@field_name' => $field_name)));
+    return $response;
   }
 
   // The user enters a comma-separated list of tags. We only autocomplete the last tag.
diff --git a/core/update.php b/core/update.php
index 968e8f4..d5b81ff 100644
--- a/core/update.php
+++ b/core/update.php
@@ -32,7 +32,7 @@
 // load that file yet as it would cause a fatal error on older versions of PHP.
 if (version_compare(PHP_VERSION, '5.3.3') < 0) {
   print 'Your PHP installation is too old. Drupal requires at least PHP 5.3.3. See the <a href="http://drupal.org/requirements">system requirements</a> page for more information.';
-  exit;
+  return;
 }
 
 /**