.../EventSubscriber/ActiveLinkResponseFilter.php   | 150 +++++++++++++++++++--
 .../system/src/Controller/SystemController.php     | 128 ------------------
 .../ActiveLinkResponseFilterTest.php}              |  17 ++-
 3 files changed, 145 insertions(+), 150 deletions(-)

diff --git a/core/lib/Drupal/Core/EventSubscriber/ActiveLinkResponseFilter.php b/core/lib/Drupal/Core/EventSubscriber/ActiveLinkResponseFilter.php
index 1f5d2ec..8f932da 100644
--- a/core/lib/Drupal/Core/EventSubscriber/ActiveLinkResponseFilter.php
+++ b/core/lib/Drupal/Core/EventSubscriber/ActiveLinkResponseFilter.php
@@ -7,6 +7,7 @@
 
 namespace Drupal\Core\EventSubscriber;
 
+use Drupal\Component\Serialization\Json;
 use Drupal\Core\Language\LanguageInterface;
 use Drupal\Core\Language\LanguageManagerInterface;
 use Drupal\Core\Path\PathMatcherInterface;
@@ -81,21 +82,144 @@ public function onResponse(FilterResponseEvent $event) {
       return;
     }
 
-    $context = [
-      'path' => $this->routeMatch->getRouteName() ? Url::fromRouteMatch($this->routeMatch)->getInternalPath() : '',
-      'front' => $this->pathMatcher->isFrontPage(),
-      'language' => $this->languageManager->getCurrentLanguage(LanguageInterface::TYPE_URL)->getId(),
-      'query' => $event->getRequest()->query->all(),
-    ];
-
+    $current_path = $this->routeMatch->getRouteName() ? Url::fromRouteMatch($this->routeMatch)->getInternalPath() : '';
     $response = $event->getResponse();
-    $html_markup = $response->getContent();
+    $response->setContent(static::setLinkActiveClass(
+      $response->getContent(),
+      $current_path,
+      $this->pathMatcher->isFrontPage(),
+      $this->languageManager->getCurrentLanguage(LanguageInterface::TYPE_URL)->getId(),
+      $event->getRequest()->query->all()
+    ));
+  }
+
+
+  /**
+   * Sets the "is-active" class on relevant links.
+   *
+   * This is a PHP implementation of the drupal.active-link JavaScript library.
+   *
+   * @param string $html_markup.
+   *   The HTML markup to update.
+   * @param string $current_path
+   *   The system path of the currently active page.
+   * @param bool $is_front
+   *   Whether the current page is the front page (which implies the current
+   *   path might also be <front>).
+   * @param string $url_language
+   *   The language code of the current URL.
+   * @param array $query
+   *   The query string for the current URL.
+   *
+   * @return string
+   *   The updated HTML markup.
+   *
+   * @todo Once a future version of PHP supports parsing HTML5 properly
+   *   (i.e. doesn't fail on https://drupal.org/comment/7938201#comment-7938201)
+   *   then we can get rid of this manual parsing and use DOMDocument instead.
+   */
+  public static function setLinkActiveClass($html_markup, $current_path, $is_front, $url_language, array $query) {
+    $search_key_current_path = 'data-drupal-link-system-path="' . $current_path . '"';
+    $search_key_front = 'data-drupal-link-system-path="&lt;front&gt;"';
+
+    // An active link's path is equal to the current path, so search the HTML
+    // for an attribute with that value.
+    $offset = 0;
+    while (strpos($html_markup, $search_key_current_path, $offset) !== FALSE || ($is_front && strpos($html_markup, $search_key_front, $offset) !== FALSE)) {
+      $pos_current_path = strpos($html_markup, $search_key_current_path, $offset);
+      $pos_front = strpos($html_markup, $search_key_front, $offset);
+
+      // Determine which of the two values is the next match: the exact path, or
+      // the <front> special case.
+      $pos_match = NULL;
+      if ($pos_front === FALSE) {
+        $pos_match = $pos_current_path;
+      }
+      elseif ($pos_current_path === FALSE) {
+        $pos_match = $pos_front;
+      }
+      elseif ($pos_current_path < $pos_front) {
+        $pos_match = $pos_current_path;
+      }
+      else {
+        $pos_match = $pos_front;
+      }
+
+      // Find beginning and ending of opening tag.
+      $pos_tag_start = NULL;
+      for ($i = $pos_match; $pos_tag_start === NULL && $i > 0; $i--) {
+        if ($html_markup[$i] === '<') {
+          $pos_tag_start = $i;
+        }
+      }
+      $pos_tag_end = NULL;
+      for ($i = $pos_match; $pos_tag_end === NULL && $i < strlen($html_markup); $i++) {
+        if ($html_markup[$i] === '>') {
+          $pos_tag_end = $i;
+        }
+      }
+
+      // Get the HTML: this will be the opening part of a single tag, e.g.:
+      //   <a href="/" data-drupal-link-system-path="&lt;front&gt;">
+      $tag = substr($html_markup, $pos_tag_start, $pos_tag_end - $pos_tag_start + 1);
+
+      // Parse it into a DOMDocument so we can reliably read and modify
+      // attributes.
+      $dom = new \DOMDocument();
+      @$dom->loadHTML('<!DOCTYPE html><html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8" /></head><body>' . $tag . '</body></html>');
+      $node = $dom->getElementsByTagName('body')->item(0)->firstChild;
+
+      // Ensure we don't set the "active" class twice on the same element.
+      $class = $node->getAttribute('class');
+      $add_active = !in_array('is-active', explode(' ', $class));
+
+      // The language of an active link is equal to the current language.
+      if ($add_active && $url_language) {
+        if ($node->hasAttribute('hreflang') && $node->getAttribute('hreflang') !== $url_language) {
+          $add_active = FALSE;
+        }
+      }
+      // The query parameters of an active link are equal to the current
+      // parameters.
+      if ($add_active) {
+        if ($query) {
+          if (!$node->hasAttribute('data-drupal-link-query') || $node->getAttribute('data-drupal-link-query') !== Json::encode($query)) {
+            $add_active = FALSE;
+          }
+        }
+        else {
+          if ($node->hasAttribute('data-drupal-link-query')) {
+            $add_active = FALSE;
+          }
+        }
+      }
+
+      // Only if the path, the language and the query match, we set the
+      // "is-active" class.
+      if ($add_active) {
+        if (strlen($class) > 0) {
+          $class .= ' ';
+        }
+        $class .= 'is-active';
+        $node->setAttribute('class', $class);
+
+        // Get the updated tag.
+        $updated_tag = $dom->saveXML($node, LIBXML_NOEMPTYTAG);
+        // saveXML() added a closing tag, remove it.
+        $updated_tag = substr($updated_tag, 0, strrpos($updated_tag, '<'));
+
+        $html_markup = str_replace($tag, $updated_tag, $html_markup);
+
+        // Ensure we only search the remaining HTML.
+        $offset = $pos_tag_end - strlen($tag) + strlen($updated_tag);
+      }
+      else {
+        // Ensure we only search the remaining HTML.
+        $offset = $pos_tag_end + 1;
+      }
+    }
 
-    $element = [
-      '#markup' => $html_markup,
-    ];
-    $element = \Drupal\system\Controller\SystemController::setLinkActiveClass($element, $context);
-    $response->setContent($element['#markup']);
+    return $html_markup;
   }
 
   /**
diff --git a/core/modules/system/src/Controller/SystemController.php b/core/modules/system/src/Controller/SystemController.php
index 90c260c..13e2f73 100644
--- a/core/modules/system/src/Controller/SystemController.php
+++ b/core/modules/system/src/Controller/SystemController.php
@@ -328,132 +328,4 @@ public function themesPage() {
     return $build;
   }
 
-  /**
-   * #post_render_cache callback; sets the "is-active" class on relevant links.
-   *
-   * This is a PHP implementation of the drupal.active-link JavaScript library.
-   *
-   * @param array $element
-   *  A renderable array with the following keys:
-   *    - #markup
-   *    - #attached
-   * @param array $context
-   *   An array with the following keys:
-   *   - path: the system path of the currently active page
-   *   - front: whether the current page is the front page (which implies the
-   *     current path might also be <front>)
-   *   - language: the language code of the currently active page
-   *   - query: the query string for the currently active page
-   *
-   * @return array
-   *   The updated renderable array.
-   *
-   * @todo Once a future version of PHP supports parsing HTML5 properly
-   *   (i.e. doesn't fail on https://drupal.org/comment/7938201#comment-7938201)
-   *   then we can get rid of this manual parsing and use DOMDocument instead.
-   */
-  public static function setLinkActiveClass(array $element, array $context) {
-    $search_key_current_path = 'data-drupal-link-system-path="' . $context['path'] . '"';
-    $search_key_front = 'data-drupal-link-system-path="&lt;front&gt;"';
-
-    // An active link's path is equal to the current path, so search the HTML
-    // for an attribute with that value.
-    $offset = 0;
-    while (strpos($element['#markup'], $search_key_current_path, $offset) !== FALSE || ($context['front'] && strpos($element['#markup'], $search_key_front, $offset) !== FALSE)) {
-      $pos_current_path = strpos($element['#markup'], $search_key_current_path, $offset);
-      $pos_front = strpos($element['#markup'], $search_key_front, $offset);
-
-      // Determine which of the two values is the next match: the exact path, or
-      // the <front> special case.
-      $pos_match = NULL;
-      if ($pos_front === FALSE) {
-        $pos_match = $pos_current_path;
-      }
-      elseif ($pos_current_path === FALSE) {
-        $pos_match = $pos_front;
-      }
-      elseif ($pos_current_path < $pos_front) {
-        $pos_match = $pos_current_path;
-      }
-      else {
-        $pos_match = $pos_front;
-      }
-
-      // Find beginning and ending of opening tag.
-      $pos_tag_start = NULL;
-      for ($i = $pos_match; $pos_tag_start === NULL && $i > 0; $i--) {
-        if ($element['#markup'][$i] === '<') {
-          $pos_tag_start = $i;
-        }
-      }
-      $pos_tag_end = NULL;
-      for ($i = $pos_match; $pos_tag_end === NULL && $i < strlen($element['#markup']); $i++) {
-        if ($element['#markup'][$i] === '>') {
-          $pos_tag_end = $i;
-        }
-      }
-
-      // Get the HTML: this will be the opening part of a single tag, e.g.:
-      //   <a href="/" data-drupal-link-system-path="&lt;front&gt;">
-      $tag = substr($element['#markup'], $pos_tag_start, $pos_tag_end - $pos_tag_start + 1);
-
-      // Parse it into a DOMDocument so we can reliably read and modify
-      // attributes.
-      $dom = new \DOMDocument();
-      @$dom->loadHTML('<!DOCTYPE html><html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8" /></head><body>' . $tag . '</body></html>');
-      $node = $dom->getElementsByTagName('body')->item(0)->firstChild;
-
-      // Ensure we don't set the "active" class twice on the same element.
-      $class = $node->getAttribute('class');
-      $add_active = !in_array('is-active', explode(' ', $class));
-
-      // The language of an active link is equal to the current language.
-      if ($add_active && $context['language']) {
-        if ($node->hasAttribute('hreflang') && $node->getAttribute('hreflang') !== $context['language']) {
-          $add_active = FALSE;
-        }
-      }
-      // The query parameters of an active link are equal to the current
-      // parameters.
-      if ($add_active) {
-        if ($context['query']) {
-          if (!$node->hasAttribute('data-drupal-link-query') || $node->getAttribute('data-drupal-link-query') !== Json::encode($context['query'])) {
-            $add_active = FALSE;
-          }
-        }
-        else {
-          if ($node->hasAttribute('data-drupal-link-query')) {
-            $add_active = FALSE;
-          }
-        }
-      }
-
-      // Only if the path, the language and the query match, we set the
-      // "is-active" class.
-      if ($add_active) {
-        if (strlen($class) > 0) {
-          $class .= ' ';
-        }
-        $class .= 'is-active';
-        $node->setAttribute('class', $class);
-
-        // Get the updated tag.
-        $updated_tag = $dom->saveXML($node, LIBXML_NOEMPTYTAG);
-        // saveXML() added a closing tag, remove it.
-        $updated_tag = substr($updated_tag, 0, strrpos($updated_tag, '<'));
-
-        $element['#markup'] = str_replace($tag, $updated_tag, $element['#markup']);
-
-        // Ensure we only search the remaining HTML.
-        $offset = $pos_tag_end - strlen($tag) + strlen($updated_tag);
-      }
-      else {
-        // Ensure we only search the remaining HTML.
-        $offset = $pos_tag_end + 1;
-      }
-    }
-
-    return $element;
-  }
-
 }
diff --git a/core/modules/system/tests/src/Unit/Controller/SystemControllerTest.php b/core/tests/Drupal/Tests/Core/EventSubscriber/ActiveLinkResponseFilterTest.php
similarity index 97%
rename from core/modules/system/tests/src/Unit/Controller/SystemControllerTest.php
rename to core/tests/Drupal/Tests/Core/EventSubscriber/ActiveLinkResponseFilterTest.php
index 173a430..b79428d 100644
--- a/core/modules/system/tests/src/Unit/Controller/SystemControllerTest.php
+++ b/core/tests/Drupal/Tests/Core/EventSubscriber/ActiveLinkResponseFilterTest.php
@@ -2,27 +2,26 @@
 
 /**
  * @file
- * Contains \Drupal\Tests\system\Unit\Controller\SystemControllerTest.
+ * Contains \Drupal\Tests\Core\EventSubscriber\ActiveLinkResponseFilterTest.
  */
 
-namespace Drupal\Tests\system\Unit\Controller;
+namespace Drupal\Tests\Core\EventSubscriber;
 
 use Drupal\Component\Serialization\Json;
-use Symfony\Component\HttpFoundation\Request;
+use Drupal\Core\EventSubscriber\ActiveLinkResponseFilter;
 use Drupal\Core\Template\Attribute;
-use Drupal\system\Controller\SystemController;
 use Drupal\Tests\UnitTestCase;
 
 /**
- * @coversDefaultClass \Drupal\system\Controller\SystemController
- * @group system
+ * @coversDefaultClass \Drupal\Core\EventSubscriber\ActiveLinkResponseFilter
+ * @group EventSubscriber
  */
-class SystemControllerTest extends UnitTestCase {
+class ActiveLinkResponseFilterTest extends UnitTestCase {
 
   /**
    * Provides test data for testSetLinkActiveClass().
    *
-   * @see \Drupal\system\Controller\SystemController::setLinkActiveClass()
+   * @see \Drupal\Core\EventSubscriber\ActiveLinkResponseFilter::setLinkActiveClass()
    */
   public function providerTestSetLinkActiveClass() {
     // Define all the variations that *don't* affect whether or not an
@@ -350,7 +349,7 @@ public function providerTestSetLinkActiveClass() {
    * @covers ::setLinkActiveClass
    */
   public function testSetLinkActiveClass(array $element, array $context, $expected_element) {
-    $this->assertSame($expected_element, SystemController::setLinkActiveClass($element, $context));
+    $this->assertSame($expected_element['#markup'], ActiveLinkResponseFilter::setLinkActiveClass($element['#markup'], $context['path'], $context['front'], $context['language'], $context['query']));
   }
 
 }