diff --git a/core/core.services.yml b/core/core.services.yml
index 183f7e2..1c31e28 100644
--- a/core/core.services.yml
+++ b/core/core.services.yml
@@ -560,8 +560,8 @@ services:
     tags:
       - { name: event_subscriber }
     arguments: ['@settings']
-  ajax_response_subscriber:
-    class: Drupal\Core\EventSubscriber\AjaxResponseSubscriber
+  ajax_subscriber:
+    class: Drupal\Core\EventSubscriber\AjaxSubscriber
     tags:
       - { name: event_subscriber }
   route_enhancer.param_conversion:
diff --git a/core/lib/Drupal/Component/Utility/Html.php b/core/lib/Drupal/Component/Utility/Html.php
index 04ce629..106e9d6 100644
--- a/core/lib/Drupal/Component/Utility/Html.php
+++ b/core/lib/Drupal/Component/Utility/Html.php
@@ -35,6 +35,13 @@ class Html {
   protected static $seenIds;
 
   /**
+   * Contains the current AJAX HTML IDs.
+   *
+   * @var array
+   */
+  protected static $ajaxHTMLIDs;
+
+  /**
    * Prepares a string for use as a valid class name.
    *
    * Do not pass one string containing multiple classes as they will be
@@ -95,6 +102,16 @@ public static function cleanCssIdentifier($identifier, array $filter = array(
   }
 
   /**
+   * Sets the AJAX HTML IDs.
+   *
+   * @param array $ajax_html_ids
+   *   The AJAX HTML IDs, probably coming from the current request.
+   */
+  public static function setAjaxHtmlIds(array $ajax_html_ids = []) {
+    static::$ajaxHTMLIDs = $ajax_html_ids;
+  }
+
+  /**
    * Prepares a string for use as a valid HTML ID and guarantees uniqueness.
    *
    * This function ensures that each passed HTML ID value only exists once on
@@ -137,7 +154,7 @@ public static function getUniqueId($id) {
       // normally not recommended as it could open up security risks, but
       // because the raw POST data is cast to a number before being returned by
       // this function, this usage is safe.
-      if (empty($_POST['ajax_html_ids'])) {
+      if (empty(static::$ajaxHTMLIDs)) {
         static::$seenIdsInit = array();
       }
       else {
@@ -146,7 +163,7 @@ public static function getUniqueId($id) {
         // that requested id. $_POST['ajax_html_ids'] contains the ids as they
         // were returned by this function, potentially with the appended
         // counter, so we parse that to reconstruct the $seen_ids array.
-        $ajax_html_ids = explode(' ', $_POST['ajax_html_ids']);
+        $ajax_html_ids = explode(' ', static::$ajaxHTMLIDs);
         foreach ($ajax_html_ids as $seen_id) {
           // We rely on '--' being used solely for separating a base id from the
           // counter, which this function ensures when returning an id.
diff --git a/core/lib/Drupal/Core/EventSubscriber/AjaxResponseSubscriber.php b/core/lib/Drupal/Core/EventSubscriber/AjaxResponseSubscriber.php
index cf4dca1..3dd052e 100644
--- a/core/lib/Drupal/Core/EventSubscriber/AjaxResponseSubscriber.php
+++ b/core/lib/Drupal/Core/EventSubscriber/AjaxResponseSubscriber.php
@@ -2,18 +2,29 @@
 
 /**
  * @file
- * Contains \Drupal\Core\EventSubscriber\AjaxResponseSubscriber.
+ * Contains \Drupal\Core\EventSubscriber\AjaxSubscriber.
  */
 
 namespace Drupal\Core\EventSubscriber;
 
+use Drupal\Component\Utility\Html;
 use Drupal\Core\Ajax\AjaxResponse;
 use Symfony\Component\EventDispatcher\EventSubscriberInterface;
 use Symfony\Component\HttpKernel\Event\FilterResponseEvent;
-use Symfony\Component\HttpKernel\HttpKernelInterface;
+use Symfony\Component\HttpKernel\Event\GetResponseEvent;
 use Symfony\Component\HttpKernel\KernelEvents;
 
-class AjaxResponseSubscriber implements EventSubscriberInterface {
+class AjaxSubscriber implements EventSubscriberInterface {
+
+  /**
+   * Sets the AJAX HTML IDs from the current request.
+   *
+   * @param \Symfony\Component\HttpKernel\Event\GetResponseEvent $event
+   *   The response event, which contains the current request.
+   */
+  public function onRequest(GetResponseEvent $event) {
+    Html::setAjaxHtmlIds($event->getRequest()->request->get('ajax_html_ids', []));
+  }
 
   /**
    * Renders the ajax commands right before preparing the result.
@@ -33,6 +44,7 @@ public function onResponse(FilterResponseEvent $event) {
    */
   public static function getSubscribedEvents() {
     $events[KernelEvents::RESPONSE][] = array('onResponse', -100);
+    $events[KernelEvents::REQUEST][] = array('onREquest', 50);
 
     return $events;
   }
diff --git a/core/lib/Drupal/Core/Form/FormBuilder.php b/core/lib/Drupal/Core/Form/FormBuilder.php
index bb88b0c..e1bcbe3 100644
--- a/core/lib/Drupal/Core/Form/FormBuilder.php
+++ b/core/lib/Drupal/Core/Form/FormBuilder.php
@@ -1136,31 +1136,6 @@ protected function getElementInfo($type) {
   }
 
   /**
-   * Wraps drupal_html_class().
-   *
-   * @return string
-   */
-  protected function drupalHtmlClass($class) {
-    return drupal_html_class($class);
-  }
-
-  /**
-   * Wraps drupal_html_id().
-   *
-   * @return string
-   */
-  protected function drupalHtmlId($id) {
-    return drupal_html_id($id);
-  }
-
-  /**
-   * Wraps drupal_static_reset().
-   */
-  protected function drupalStaticReset($name = NULL) {
-    drupal_static_reset($name);
-  }
-
-  /**
    * Gets the current active user.
    *
    * @return \Drupal\Core\Session\AccountInterface
diff --git a/core/tests/Drupal/Tests/Core/Form/FormTestBase.php b/core/tests/Drupal/Tests/Core/Form/FormTestBase.php
index 108f35a..1f09bbe 100644
--- a/core/tests/Drupal/Tests/Core/Form/FormTestBase.php
+++ b/core/tests/Drupal/Tests/Core/Form/FormTestBase.php
@@ -7,7 +7,8 @@
 
 namespace Drupal\Tests\Core\Form {
 
-use Drupal\Core\Form\FormBuilder;
+  use Drupal\Component\Utility\Html;
+  use Drupal\Core\Form\FormBuilder;
 use Drupal\Core\Form\FormInterface;
 use Drupal\Core\Form\FormStateInterface;
 use Drupal\Core\Session\AccountInterface;
@@ -193,7 +194,7 @@ protected function setUp() {
    * {@inheritdoc}
    */
   protected function tearDown() {
-    $this->formBuilder->drupalStaticReset();
+    Html::resetSeenIds();
   }
 
   /**