diff --git a/core/core.services.yml b/core/core.services.yml
index 9fb9ebd..250e387 100644
--- a/core/core.services.yml
+++ b/core/core.services.yml
@@ -261,8 +261,18 @@ services:
     class: Drupal\Core\Routing\Enhancer\AjaxEnhancer
     arguments: ['@content_negotiation']
     tags:
+      - { name: route_enhancer, priority: 40 }
+      - { name: legacy_route_enhancer, priority: 40 }
+  route_enhancer.dialog:
+    class: Drupal\Core\Routing\Enhancer\DialogEnhancer
+    arguments: ['@content_negotiation']
+    tags:
+      - { name: route_enhancer, priority: 30 }
+  route_enhancer.modal:
+    class: Drupal\Core\Routing\Enhancer\ModalEnhancer
+    arguments: ['@content_negotiation']
+    tags:
       - { name: route_enhancer, priority: 20 }
-      - { name: legacy_route_enhancer, priority: 20 }
   route_enhancer.form:
     class: Drupal\Core\Routing\Enhancer\FormEnhancer
     arguments: ['@content_negotiation']
diff --git a/core/includes/ajax.inc b/core/includes/ajax.inc
index ce56aa7..42843fa 100644
--- a/core/includes/ajax.inc
+++ b/core/includes/ajax.inc
@@ -519,6 +519,7 @@ function ajax_process_form($element, &$form_state) {
  *   - #ajax['wrapper']
  *   - #ajax['parameters']
  *   - #ajax['effect']
+ *   - #ajax['accepts']
  *
  * @return
  *   The processed element with the necessary JavaScript attached to it.
@@ -607,6 +608,7 @@ function ajax_pre_render_element($element) {
     $settings += array(
       'path' => isset($settings['callback']) ? 'system/ajax' : NULL,
       'options' => array(),
+      'accepts' => 'application/vnd.drupal-ajax'
     );
 
     // @todo Legacy support. Remove in Drupal 8.
diff --git a/core/lib/Drupal/Core/Ajax/AjaxSubscriber.php b/core/lib/Drupal/Core/Ajax/AjaxSubscriber.php
index 5ef67c0..414511f 100644
--- a/core/lib/Drupal/Core/Ajax/AjaxSubscriber.php
+++ b/core/lib/Drupal/Core/Ajax/AjaxSubscriber.php
@@ -27,6 +27,8 @@ public function onKernelRequest(GetResponseEvent $event) {
     // @todo Refactor 'drupal_ajax' to just 'ajax' once all Ajax is converted to
     // Drupal 8's API.
     $request->setFormat('drupal_ajax', 'application/vnd.drupal-ajax');
+    $request->setFormat('drupal_dialog', 'application/vnd.drupal-dialog');
+    $request->setFormat('drupal_modal', 'application/vnd.drupal-modal');
   }
 
   /**
@@ -40,4 +42,4 @@ static function getSubscribedEvents(){
     return $events;
   }
 
-}
\ No newline at end of file
+}
diff --git a/core/lib/Drupal/Core/Ajax/DialogController.php b/core/lib/Drupal/Core/Ajax/DialogController.php
new file mode 100644
index 0000000..1ff011e
--- /dev/null
+++ b/core/lib/Drupal/Core/Ajax/DialogController.php
@@ -0,0 +1,135 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\core\Ajax\DialogController
+ */
+
+namespace Drupal\core\Ajax;
+
+use Drupal\Core\ControllerInterface;
+use Drupal\Core\Ajax\AjaxResponse;
+use Drupal\Core\Ajax\OpenModalDialogCommand;
+use Drupal\Core\Ajax\OpenDialogCommand;
+use Symfony\Component\HttpFoundation\Request;
+use Symfony\Component\DependencyInjection\ContainerAware;
+
+class DialogController extends ContainerAware {
+
+  /**
+   * Utility to get forward request to a subrequest.
+   *
+   * @param Request $request
+   *   The request object.
+   * @param callable $content
+   *   The body content callable that contains the body region of this page.
+   *
+   * @return \Symfony\Component\HttpFoundation\Response
+   *   A response object.
+   */
+  protected function forward($request, $content) {
+    // @todo When we have a Generator, we can replace the forward() call with
+    // a render() call, which would handle ESI and hInclude as well.  That will
+    // require an _internal route.  For examples, see:
+    // https://github.com/symfony/symfony/blob/master/src/Symfony/Bundle/FrameworkBundle/Resources/config/routing/internal.xml
+    // https://github.com/symfony/symfony/blob/master/src/Symfony/Bundle/FrameworkBundle/Controller/InternalController.php
+    $attributes = clone $request->attributes;
+    // We need to clean up the derived information and such so that the
+    // subrequest can be processed properly without leaking data through.
+    $attributes->remove('system_path');
+    $attributes->remove('_legacy');
+
+    // Remove the accept header so the subrequest does not end up back in this
+    // controller.
+    $request->headers->remove('accept');
+
+    return $this->container->get('http_kernel')->forward($content, $attributes->all(), $request->query->all());
+  }
+
+  /**
+   * Wrapper to display content in a modal dialog.
+   *
+   * @param Request $request
+   *   The request object.
+   * @param callable $_content
+   *   The body content callable that contains the body region of this page.
+   *
+   * @return \Drupal\Core\Ajax\AjaxResponse
+   *   AjaxResponse to return the content wrapper in a modal dialog.
+   */
+  public function modal(Request $request, $_content) {
+    return $this->dialog($request, $_content, TRUE);
+  }
+
+  /**
+   * Wrapper to display content in a dialog.
+   *
+   * @param Request $request
+   *   The request object.
+   * @param callable $_content
+   *   The body content callable that contains the body region of this page.
+   * @param bool $modal
+   *   (optional) TRUE to render a modal dialog. Defaults to FALSE.
+   *
+   * @return \Drupal\Core\Ajax\AjaxResponse
+   *   AjaxResponse to return the content wrapper in a dialog.
+   */
+  public function dialog(Request $request, $_content, $modal = FALSE) {
+    $subrequest = $this->forward($request, $_content);
+    if ($subrequest->isOk()) {
+      $output = $subrequest->getContent();
+      // A page callback could return a render array or a string.
+      if (is_array($output)) {
+        // Subrequest has set dialog title in the output.
+        if (isset($output['title'])) {
+          $title = $output['title']['#value'];
+          unset($output['title']);
+        }
+        else {
+          // Legacy fallback.
+          // @todo remove when http://drupal.org/node/1830588 lands.
+          $title = drupal_get_title();
+        }
+        $content = drupal_render($output);
+      }
+      else {
+        // Legacy fallback.
+        // @todo remove when http://drupal.org/node/1830588 lands and all routes
+        // converted to return render arrays.
+        $title = drupal_get_title();
+        $content = $output;
+      }
+      $response = new AjaxResponse();
+      // Fetch any modal options passed in from data-dialog-options.
+      if (!($options = $request->request->get('dialog_options'))) {
+        $options = array();
+      }
+      // Set modal flag and re-use the modal id.
+      if ($modal) {
+        $options['modal'] = TRUE;
+        $target = '#drupal-modal';
+      }
+      else {
+        // Generate the target wrapper for the dialog.
+        if (isset($options['target'])) {
+          // If the target was nominated in the incoming options, use that.
+          $target = $options['target'];
+          // Ensure the target includes the #.
+          if (substr($target, 0, 1) != '#') {
+            $target = '#' . $target;
+          }
+          // This shouldn't be passed on to jQuery.ui.dialog.
+          unset($options['target']);
+        }
+        else {
+          // Generate a target based on the controller.
+          $target = '#drupal-dialog-' . drupal_html_id(drupal_clean_css_identifier(drupal_strtolower($_content)));
+        }
+      }
+      $response->addCommand(new OpenDialogCommand($target, $title, $content, $options));
+      return $response;
+    }
+    // An error occurred in the subrequest, return that.
+    return $subrequest;
+  }
+}
diff --git a/core/lib/Drupal/Core/ContentNegotiation.php b/core/lib/Drupal/Core/ContentNegotiation.php
index a2c17b5..d2ffa45 100644
--- a/core/lib/Drupal/Core/ContentNegotiation.php
+++ b/core/lib/Drupal/Core/ContentNegotiation.php
@@ -26,7 +26,7 @@ class ContentNegotiation {
    * @param Symfony\Component\HttpFoundation\Request $request
    *   The request object from which to extract the content type.
    *
-   * @return
+   * @return string
    *   The normalized type of a given request.
    */
   public function getContentType(Request $request) {
@@ -36,11 +36,12 @@ public function getContentType(Request $request) {
       return 'iframeupload';
     }
 
-    // Check all formats, it HTML is found return it.
+    // Check all formats, if priority format is found return it.
     $first_found_format = FALSE;
     foreach ($request->getAcceptableContentTypes() as $mime_type) {
       $format = $request->getFormat($mime_type);
-      if ($format === 'html' || $format === 'drupal_ajax') {
+      $priority = array('html', 'drupal_ajax', 'drupal_modal', 'drupal_dialog');
+      if (in_array($format, $priority, TRUE)) {
         return $format;
       }
       if (!is_null($format) && !$first_found_format) {
diff --git a/core/lib/Drupal/Core/Routing/Enhancer/DialogEnhancer.php b/core/lib/Drupal/Core/Routing/Enhancer/DialogEnhancer.php
new file mode 100644
index 0000000..ded14e0
--- /dev/null
+++ b/core/lib/Drupal/Core/Routing/Enhancer/DialogEnhancer.php
@@ -0,0 +1,62 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Core\Routing\Enhancer\DialogEnhancer.
+ */
+
+namespace Drupal\Core\Routing\Enhancer;
+
+use Symfony\Component\HttpFoundation\Request;
+use Symfony\Cmf\Component\Routing\Enhancer\RouteEnhancerInterface;
+use Drupal\Core\ContentNegotiation;
+
+/**
+ * Enhances a route to use the DialogController for matching requests.
+ */
+class DialogEnhancer implements RouteEnhancerInterface {
+
+  /**
+   * Content negotiation library.
+   *
+   * @var \Drupal\CoreContentNegotiation
+   */
+  protected $negotiation;
+
+  /**
+   * Content type this enhancer targets.
+   *
+   * @var string
+   */
+  protected $targetContentType = 'drupal_dialog';
+
+  /**
+   * Controller to route matching requests to.
+   *
+   * @var string
+   */
+  protected $controller = '\Drupal\Core\Ajax\DialogController::dialog';
+
+  /**
+   * Constructs a new \Drupal\Core\Routing\Enhancer\AjaxEnhancer object.
+   *
+   * @param \Drupal\Core\ContentNegotiation $negotiation
+   *   The Content Negotiation service.
+   */
+  public function __construct(ContentNegotiation $negotiation) {
+    $this->negotiation = $negotiation;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function enhance(array $defaults, Request $request) {
+    if ($this->negotiation->getContentType($request) == $this->targetContentType) {
+      if (empty($defaults['_content']) && !empty($defaults['controller'])) {
+        // Pass the controller on as content.
+        $defaults['_content'] = $defaults['_controller'];
+      }
+      $defaults['_controller'] = $this->controller;
+    }
+  }
+}
diff --git a/core/lib/Drupal/Core/Routing/Enhancer/ModalEnhancer.php b/core/lib/Drupal/Core/Routing/Enhancer/ModalEnhancer.php
new file mode 100644
index 0000000..c492e8c
--- /dev/null
+++ b/core/lib/Drupal/Core/Routing/Enhancer/ModalEnhancer.php
@@ -0,0 +1,32 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Core\Routing\Enhancer\ModalEnhancer.
+ */
+
+namespace Drupal\Core\Routing\Enhancer;
+
+use Symfony\Component\HttpFoundation\Request;
+use Drupal\Core\ContentNegotiation;
+
+/**
+ * Enhances a route to use the DialogController for matching requests.
+ */
+class ModalEnhancer extends DialogEnhancer {
+
+  /**
+   * Content type this enhancer targets.
+   *
+   * @var string
+   */
+  protected $targetContentType = 'drupal_modal';
+
+  /**
+   * Controller to route matching requests to.
+   *
+   * @var string
+   */
+  protected $controller = '\Drupal\Core\Ajax\DialogController::modal';
+
+}
diff --git a/core/misc/ajax.js b/core/misc/ajax.js
index 45a5bac..1449c4a 100644
--- a/core/misc/ajax.js
+++ b/core/misc/ajax.js
@@ -52,6 +52,12 @@ Drupal.behaviors.AJAX = {
         element_settings.url = $(this).attr('href');
         element_settings.event = 'click';
       }
+      if ($(this).data('accepts')) {
+        element_settings.accepts = $(this).data('accepts');
+      }
+      if ($(this).data('dialog-options')) {
+        element_settings.dialog = $(this).data('dialog-options');
+      }
       var base = $(this).attr('id');
       Drupal.ajax[base] = new Drupal.ajax(base, this, element_settings);
     });
@@ -200,11 +206,15 @@ Drupal.ajax = function (base, element, element_settings) {
     },
     dataType: 'json',
     accepts: {
-      json: 'application/vnd.drupal-ajax'
+      json: element_settings.accepts || 'application/vnd.drupal-ajax'
     },
     type: 'POST'
   };
 
+  if (element_settings.dialog) {
+    ajax.options.data['dialog_options'] = element_settings.dialog;
+  }
+
   // Bind the ajaxSubmit function to the element event.
   $(ajax.element).bind(element_settings.event, function (event) {
     return ajax.eventResponse(this, event);
diff --git a/core/modules/config/config.admin.inc b/core/modules/config/config.admin.inc
index 51e5f8e..4e12ea4 100644
--- a/core/modules/config/config.admin.inc
+++ b/core/modules/config/config.admin.inc
@@ -77,6 +77,10 @@ function config_admin_sync_form(array &$form, array &$form_state, StorageInterfa
         'href' => 'admin/config/development/sync/diff/' . $config_file,
         'attributes' => array(
           'class' => array('use-ajax'),
+          'data-accepts' => 'application/vnd.drupal-modal',
+          'data-dialog-options' => json_encode(array(
+            'width' => 500
+          )),
         ),
       );
       $form[$config_change_type]['list']['#rows'][] = array(
@@ -121,7 +125,7 @@ function config_admin_import_form_submit($form, &$form_state) {
   if (!lock()->lockMayBeAvailable(CONFIG_IMPORT_LOCK)) {
     drupal_set_message(t('Another request may be synchronizing configuration already.'));
   }
-  else if (config_import()) {
+  elseif (config_import()) {
     // Once a sync completes, we empty the staging directory. This prevents
     // changes from being accidentally overwritten by stray files getting
     // imported later.
@@ -138,69 +142,3 @@ function config_admin_import_form_submit($form, &$form_state) {
     drupal_set_message(t('The import failed due to an error. Any errors have been logged.'), 'error');
   }
 }
-
-/**
- * Page callback: Shows diff of specificed configuration file.
- *
- * @param string $config_file
- *   The name of the configuration file.
- *
- * @return string
- *   Table showing a two-way diff between the active and staged configuration.
- */
-function config_admin_diff_page($config_file) {
-  // Retrieve a list of differences between last known state and active store.
-  $source_storage = drupal_container()->get('config.storage.staging');
-  $target_storage = drupal_container()->get('config.storage');
-
-  // Add the CSS for the inline diff.
-  $output['#attached']['css'][] = drupal_get_path('module', 'system') . '/system.diff.css';
-
-  $diff = config_diff($target_storage, $source_storage, $config_file);
-  $formatter = new DrupalDiffFormatter();
-  $formatter->show_header = FALSE;
-
-  $variables = array(
-    'header' => array(
-      array('data' => t('Old'), 'colspan' => '2'),
-      array('data' => t('New'), 'colspan' => '2'),
-    ),
-    'rows' => $formatter->format($diff),
-  );
-
-  $output['diff'] = array(
-    '#markup' => theme('table', $variables),
-  );
-
-  $output['back'] = array(
-    '#type' => 'link',
-    '#title' => "Back to 'Synchronize configuration' page.",
-    '#href' => 'admin/config/development/sync',
-  );
-
-  $title = t('View changes of @config_file', array('@config_file' => $config_file));
-
-  // Return AJAX requests as a dialog.
-  // @todo: Set up separate content callbacks for the non-JS and dialog versions
-  // of this page using the router system. See http://drupal.org/node/1944472.
-  if (Drupal::request()->isXmlHttpRequest()) {
-    // Add class to the close link.
-    $output['back']['#attributes']['class'][] = 'dialog-cancel';
-
-    $dialog_content = drupal_render($output);
-    $response = new AjaxResponse();
-    $response->addCommand(new OpenModalDialogCommand($title, $dialog_content, array('width' => '700')));
-    return $response;
-  }
-  // Otherwise show the page title as an element.
-  else {
-    $output['title'] = array(
-      '#theme' => 'html_tag',
-      '#tag' => 'h3',
-      '#value' => $title,
-      '#weight' => -10,
-    );
-  }
-
-  return $output;
-}
diff --git a/core/modules/config/config.module b/core/modules/config/config.module
index e3b6027..651e387 100644
--- a/core/modules/config/config.module
+++ b/core/modules/config/config.module
@@ -48,18 +48,9 @@ function config_menu() {
     'access arguments' => array('synchronize configuration'),
     'file' => 'config.admin.inc',
   );
-  $items['admin/config/development/sync/diff/%'] = array(
-    'title' => 'Configuration file diff',
-    'description' => 'Diff between active and staged configuraiton.',
-    'page callback' => 'config_admin_diff_page',
-    'page arguments' => array(5),
-    'access arguments' => array('synchronize configuration'),
-    'file' => 'config.admin.inc',
-  );
   $items['admin/config/development/sync/import'] = array(
     'title' => 'Import',
     'type' => MENU_DEFAULT_LOCAL_TASK,
   );
   return $items;
 }
-
diff --git a/core/modules/config/config.routing.yml b/core/modules/config/config.routing.yml
new file mode 100644
index 0000000..505539d
--- /dev/null
+++ b/core/modules/config/config.routing.yml
@@ -0,0 +1,6 @@
+config_diff:
+  pattern: '/admin/config/development/sync/diff/{config_file}'
+  defaults:
+    _content: '\Drupal\config\Controller\ConfigController::diff'
+  requirements:
+    _permission: 'synchronize configuration'
diff --git a/core/modules/config/lib/Drupal/config/Controller/ConfigController.php b/core/modules/config/lib/Drupal/config/Controller/ConfigController.php
new file mode 100644
index 0000000..471d656
--- /dev/null
+++ b/core/modules/config/lib/Drupal/config/Controller/ConfigController.php
@@ -0,0 +1,102 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\config\Controller\ConfigController
+ */
+
+namespace Drupal\config\Controller;
+
+use Drupal\Core\ControllerInterface;
+use Drupal\Core\Config\StorageInterface;
+use Drupal\Core\Ajax\AjaxResponse;
+use Drupal\Core\Ajax\OpenModalDialogCommand;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+class ConfigController implements ControllerInterface {
+
+  /**
+   * The target storage.
+   *
+   * @var \Drupal\Core\Config\StorageInterface;
+   */
+  protected $targetStorage;
+
+  /**
+   * The source storage.
+   *
+   * @var \Drupal\Core\Config\StorageInterface;
+   */
+  protected $sourceStorage;
+
+  /**
+   * Inject the target and source config storage services.
+   */
+  public static function create(ContainerInterface $container) {
+    return new static($container->get('config.storage'), $container->get('config.storage.staging'));
+  }
+
+  /**
+   * Constructor.
+   *
+   * @param \Drupal\Core\Config\StorageInterface $target_storage
+   *   The target storage.
+   * @param \Drupal\Core\Config\StorageInterface $source_storage
+   *   The source storage
+   */
+  public function __construct(StorageInterface $target_storage, StorageInterface $source_storage) {
+    $this->targetStorage = $target_storage;
+    $this->sourceStorage = $source_storage;
+  }
+
+  /**
+   * Shows diff of specificed configuration file.
+   *
+   * @param string $config_file
+   *   The name of the configuration file.
+   *
+   * @return string
+   *   Table showing a two-way diff between the active and staged configuration.
+   */
+  public function diff($config_file) {
+    // Add the CSS for the inline diff.
+    $output['#attached']['css'][] = drupal_get_path('module', 'system') . '/system.diff.css';
+
+    $diff = config_diff($this->targetStorage, $this->sourceStorage, $config_file);
+    $formatter = new \DrupalDiffFormatter();
+    $formatter->show_header = FALSE;
+
+    $variables = array(
+      'header' => array(
+        array('data' => t('Old'), 'colspan' => '2'),
+        array('data' => t('New'), 'colspan' => '2'),
+      ),
+      'rows' => $formatter->format($diff),
+    );
+
+    $output['diff'] = array(
+      '#markup' => theme('table', $variables),
+    );
+
+    $output['back'] = array(
+      '#type' => 'link',
+      '#attributes' => array(
+        'class' => array(
+          'dialog-cancel',
+        ),
+      ),
+      '#title' => "Back to 'Synchronize configuration' page.",
+      '#href' => 'admin/config/development/sync',
+    );
+
+    $title = t('View changes of @config_file', array('@config_file' => $config_file));
+    $output['title'] = array(
+      '#theme' => 'html_tag',
+      '#tag' => 'h3',
+      '#value' => $title,
+      '#weight' => -10,
+    );
+
+    return $output;
+  }
+}
diff --git a/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
index a5c34b3..d79bf81 100644
--- a/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
@@ -1197,6 +1197,16 @@ protected function drupalGet($path, array $options = array(), array $headers = a
 
   /**
    * Retrieve a Drupal path or an absolute path and JSON decode the result.
+   *
+   * @param string $path
+   *   Path to request AJAX from.
+   * @param array $options
+   *   Array of options to pass to url().
+   * @param array $headers
+   *   Array of headers. Eg array('Accept: application/vnd.drupal-ajax').
+   *
+   * @return array
+   *   Decoded json.
    */
   protected function drupalGetAJAX($path, array $options = array(), array $headers = array()) {
     $headers[] = 'X-Requested-With: XMLHttpRequest';
diff --git a/core/modules/system/lib/Drupal/system/Tests/Ajax/DialogTest.php b/core/modules/system/lib/Drupal/system/Tests/Ajax/DialogTest.php
index 32aea71..7b4e093 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Ajax/DialogTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Ajax/DialogTest.php
@@ -11,6 +11,10 @@
  * Tests use of dialogs as wrappers for Ajax responses.
  */
 class DialogTest extends AjaxTestBase {
+
+  /**
+   * Declares test info.
+   */
   public static function getInfo() {
     return array(
       'name' => 'AJAX dialogs commands',
@@ -22,7 +26,7 @@ public static function getInfo() {
   /**
    * Test sending non-JS and AJAX requests to open and manipulate modals.
    */
-  function testDialog() {
+  public function testDialog() {
     // Ensure the elements render without notices or exceptions.
     $this->drupalGet('ajax-test/dialog');
 
@@ -35,8 +39,8 @@ function testDialog() {
       'settings' => NULL,
       'data' => $dialog_contents,
       'dialogOptions' => array(
-        'modal' => true,
-        'title' => 'AJAX Dialog',
+        'modal' => TRUE,
+        'title' => 'AJAX Dialog contents',
       ),
     );
     $normal_expected_response = array(
@@ -45,8 +49,8 @@ function testDialog() {
       'settings' => NULL,
       'data' => $dialog_contents,
       'dialogOptions' => array(
-        'modal' => false,
-        'title' => 'AJAX Dialog',
+        'modal' => FALSE,
+        'title' => 'AJAX Dialog contents',
       ),
     );
     $close_expected_response = array(
@@ -55,20 +59,29 @@ function testDialog() {
     );
 
     // Check that requesting a modal dialog without JS goes to a page.
-    $this->drupalGet('ajax-test/dialog-contents/nojs/1');
+    $this->drupalGet('ajax-test/dialog-contents');
     $this->assertRaw($dialog_contents, 'Non-JS modal dialog page present.');
 
     // Emulate going to the JS version of the page and check the JSON response.
-    $ajax_result = $this->drupalGetAJAX('ajax-test/dialog-contents/ajax/1');
+    $ajax_result = $this->drupalGetAJAX('ajax-test/dialog-contents', array(), array('Accept: application/vnd.drupal-modal'));
     $this->assertEqual($modal_expected_response, $ajax_result[1], 'Modal dialog JSON response matches.');
 
     // Check that requesting a "normal" dialog without JS goes to a page.
-    $this->drupalGet('ajax-test/dialog-contents/nojs');
+    $this->drupalGet('ajax-test/dialog-contents');
     $this->assertRaw($dialog_contents, 'Non-JS normal dialog page present.');
 
     // Emulate going to the JS version of the page and check the JSON response.
-    $ajax_result = $this->drupalGetAJAX('ajax-test/dialog-contents/ajax');
-    $this->assertEqual($normal_expected_response, $ajax_result[1], 'Normal dialog JSON response matches.');
+    // This needs to use WebTestBase::drupalPostAJAX() so that the correct
+    // dialog options are sent.
+    $ajax_result = $this->drupalPostAJAX('ajax-test/dialog', array(
+        // We have to mock a form element to make drupalPost submit from a link.
+        'textfield' => 'test',
+      ), array(), 'ajax-test/dialog-contents', array(), array('Accept: application/vnd.drupal-dialog'), NULL, array(
+      'submit' => array(
+        'dialog_options[target]' => 'ajax-test-dialog-wrapper-1',
+      )
+    ));
+    $this->assertEqual($normal_expected_response, $ajax_result[3], 'Normal dialog JSON response matches.');
 
     // Emulate closing the dialog via an AJAX request. There is no non-JS
     // version of this test.
diff --git a/core/modules/system/tests/modules/ajax_test/ajax_test.module b/core/modules/system/tests/modules/ajax_test/ajax_test.module
index 2ed5a16..5421f16 100644
--- a/core/modules/system/tests/modules/ajax_test/ajax_test.module
+++ b/core/modules/system/tests/modules/ajax_test/ajax_test.module
@@ -124,8 +124,11 @@ function ajax_test_dialog() {
   $build['link'] = array(
     '#type' => 'link',
     '#title' => 'Link 1 (modal)',
-    '#href' => 'ajax-test/dialog-contents/nojs/1',
-    '#attributes' => array('class' => array('use-ajax')),
+    '#href' => 'ajax-test/dialog-contents',
+    '#attributes' => array(
+      'class' => array('use-ajax'),
+      'data-accepts' => 'application/vnd.drupal-modal',
+    ),
   );
 
   // Dialog behavior applied to links rendered by theme_links().
@@ -134,18 +137,33 @@ function ajax_test_dialog() {
     '#links' => array(
       'link2' => array(
         'title' => 'Link 2 (modal)',
-        'href' => 'ajax-test/dialog-contents/nojs/1',
-        'attributes' => array('class' => array('use-ajax')),
+        'href' => 'ajax-test/dialog-contents',
+        'attributes' => array(
+          'class' => array('use-ajax'),
+          'data-accepts' => 'application/vnd.drupal-modal',
+          'data-dialog-options' => json_encode(array(
+            'width' => 400,
+          ))
+        ),
       ),
       'link3' => array(
         'title' => 'Link 3 (non-modal)',
-        'href' => 'ajax-test/dialog-contents/nojs',
-        'attributes' => array('class' => array('use-ajax')),
+        'href' => 'ajax-test/dialog-contents',
+        'attributes' => array(
+          'class' => array('use-ajax'),
+          'data-accepts' => 'application/vnd.drupal-dialog',
+          'data-dialog-options' => json_encode(array(
+            'target' => 'ajax-test-dialog-wrapper-1',
+            'width' => 800,
+          ))
+        ),
       ),
       'link4' => array(
         'title' => 'Link 4 (close non-modal if open)',
         'href' => 'ajax-test/dialog-close',
-        'attributes' => array('class' => array('use-ajax')),
+        'attributes' => array(
+          'class' => array('use-ajax'),
+        ),
       ),
     ),
   );
@@ -156,6 +174,12 @@ function ajax_test_dialog() {
  * Form builder: Renders buttons with #ajax['dialog'].
  */
 function ajax_test_dialog_form($form, &$form_state) {
+  // In order to use WebTestBase::drupalPostAJAX() to POST from a link, we need
+  // to have a dummy field we can set in WebTestBase::drupalPost() else it won't
+  // submit anything.
+  $form['textfield'] = array(
+    '#type' => 'hidden'
+  );
   $form['button1'] = array(
     '#type' => 'submit',
     '#name' => 'button1',
@@ -186,26 +210,47 @@ function ajax_test_dialog_form_submit($form, &$form_state) {
  * AJAX callback handler for ajax_test_dialog_form().
  */
 function ajax_test_dialog_form_callback_modal($form, &$form_state) {
-  return ajax_test_dialog_contents('ajax', TRUE);
+  return _ajax_test_dialog(TRUE);
 }
 
 /**
  * AJAX callback handler for ajax_test_dialog_form().
  */
 function ajax_test_dialog_form_callback_nonmodal($form, &$form_state) {
-  return ajax_test_dialog_contents('ajax', FALSE);
+  return _ajax_test_dialog(FALSE);
+}
+
+/**
+ * Util to render dialog in ajax callback.
+ *
+ * @param bool $is_modal
+ *   (optional) TRUE if modal, FALSE if plain dialog. Defaults to FALSE.
+ */
+function _ajax_test_dialog($is_modal = FALSE) {
+  $content = ajax_test_dialog_contents();
+  $response = new AjaxResponse();
+  $title = t('AJAX Dialog contents');
+  $html = drupal_render($content);
+  if ($is_modal) {
+    $response->addCommand(new OpenModalDialogCommand($title, $html));
+  }
+  else {
+    $selector = '#ajax-test-dialog-wrapper-1';
+    $response->addCommand(new OpenDialogCommand($selector, $title, $html));
+  }
+  return $response;
 }
 
 /**
  * Menu callback: Returns the contents for dialogs opened by ajax_test_dialog().
  */
-function ajax_test_dialog_contents($page_mode = 'nojs', $is_modal = 0) {
+function ajax_test_dialog_contents() {
   // This is a regular render array; the keys do not have special meaning.
   $content = array(
     'content' => array(
       '#markup' => 'Example message',
     ),
-    'cancel'=> array(
+    'cancel' => array(
       '#type' => 'link',
       '#title' => 'Cancel',
       '#href' => '',
@@ -217,22 +262,7 @@ function ajax_test_dialog_contents($page_mode = 'nojs', $is_modal = 0) {
     ),
   );
 
-  if ($page_mode === 'ajax') {
-    $response = new AjaxResponse();
-    $title = t('AJAX Dialog');
-    $html = drupal_render($content);
-    if ($is_modal) {
-      $response->addCommand(new OpenModalDialogCommand($title, $html));
-    }
-    else {
-      $selector = '#ajax-test-dialog-wrapper-1';
-      $response->addCommand(new OpenDialogCommand($selector, $title, $html));
-    }
-    return $response;
-  }
-  else {
-    return $content;
-  }
+  return $content;
 }
 
 /**