core/lib/Drupal/Core/Cache/DatabaseBackend.php     |    2 +-
 .../Core/EventSubscriber/HtmlViewSubscriber.php    |   32 +-------------------
 .../lib/Drupal/node/Tests/NodePageCacheTest.php    |    5 ++-
 .../system/Tests/Bootstrap/PageCacheTest.php       |    1 +
 .../Tests/Cache/PageCacheTagsIntegrationTest.php   |   17 +++++++----
 5 files changed, 18 insertions(+), 39 deletions(-)

diff --git a/core/lib/Drupal/Core/Cache/DatabaseBackend.php b/core/lib/Drupal/Core/Cache/DatabaseBackend.php
index 4f98c50..0647faa 100644
--- a/core/lib/Drupal/Core/Cache/DatabaseBackend.php
+++ b/core/lib/Drupal/Core/Cache/DatabaseBackend.php
@@ -366,7 +366,7 @@ public function garbageCollection() {
    * @return array
    *   An indexed array of flattened tag identifiers.
    */
-  protected function flattenTags(array $tags) {
+  public static function flattenTags(array $tags) {
     if (isset($tags[0])) {
       return $tags;
     }
diff --git a/core/lib/Drupal/Core/EventSubscriber/HtmlViewSubscriber.php b/core/lib/Drupal/Core/EventSubscriber/HtmlViewSubscriber.php
index e5c5e57..5945a1a 100644
--- a/core/lib/Drupal/Core/EventSubscriber/HtmlViewSubscriber.php
+++ b/core/lib/Drupal/Core/EventSubscriber/HtmlViewSubscriber.php
@@ -77,7 +77,7 @@ public function onHtmlPage(GetResponseForControllerResultEvent $event) {
       // recommended.
       $response = new Response((string) $this->pageRenderer->render($page), $page->getStatusCode());
       if ($tags = $page->getCacheTags()) {
-        $response->headers->set('X-Drupal-Cache-Tags', $this->flattenTags($tags));
+        $response->headers->set('X-Drupal-Cache-Tags', implode(' ', \Drupal\Core\Cache\DatabaseBackend::flattenTags($tags)));
       }
       if ($keys = $page->getCacheKeys()) {
         $response->headers->set('cache_keys', serialize($keys));
@@ -93,36 +93,6 @@ public function onHtmlPage(GetResponseForControllerResultEvent $event) {
   }
 
   /**
-   * 'Flattens' a cache tags array into a space-separated flattened tags list.
-   *
-   * @param array $tags
-   *   Associative array of tags to flatten.
-   *
-   * @return string
-   *   A space-separated list of flattened tags.
-   *
-   * @see \Drupal\Core\Cache\DatabaseBackend::flattenTags()
-   */
-  protected function flattenTags(array $tags) {
-    if (isset($tags[0])) {
-      return $tags;
-    }
-
-    $flat_tags = array();
-    foreach ($tags as $namespace => $values) {
-      if (is_array($values)) {
-        foreach ($values as $value) {
-          $flat_tags[] = "$namespace:$value";
-        }
-      }
-      else {
-        $flat_tags[] = "$namespace:$values";
-      }
-    }
-    return implode(' ', $flat_tags);
-  }
-
-  /**
    * Registers the methods in this class that should be listeners.
    *
    * @return array
diff --git a/core/modules/node/lib/Drupal/node/Tests/NodePageCacheTest.php b/core/modules/node/lib/Drupal/node/Tests/NodePageCacheTest.php
index 0fc6ed2..70c3dc2 100644
--- a/core/modules/node/lib/Drupal/node/Tests/NodePageCacheTest.php
+++ b/core/modules/node/lib/Drupal/node/Tests/NodePageCacheTest.php
@@ -59,7 +59,10 @@ public function testNodeDelete() {
     $cid_parts = array(url($node_path, array('absolute' => TRUE)), 'html');
     $cid = sha1(implode(':', $cid_parts));
     $cache_entry = \Drupal::cache('page')->get($cid);
-    $this->assertIdentical($cache_entry->tags, array('content:1', 'node_view:' . $node_id,  'node:' . $node_id, 'user:' . $author->id(), 'filter_format:plain_text'));
+    sort($cache_entry->tags);
+    $expected_tags = array('content:1', 'node_view:' . $node_id,  'node:' . $node_id, 'user:' . $author->id(), 'filter_format:plain_text');
+    sort($expected_tags);
+    $this->assertIdentical($cache_entry->tags, $expected_tags);
 
     // Login and delete the node.
     $this->drupalLogin($this->adminUser);
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 3ca1f19..ec596fa 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Bootstrap/PageCacheTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Bootstrap/PageCacheTest.php
@@ -65,6 +65,7 @@ function testPageCacheTags() {
     $cid_parts = array(url($path, array('absolute' => TRUE)), 'html');
     $cid = sha1(implode(':', $cid_parts));
     $cache_entry = \Drupal::cache('page')->get($cid);
+    sort($cache_entry->tags);
     $this->assertIdentical($cache_entry->tags, array('content:1', 'system_test_cache_tags_page:1'));
 
     Cache::invalidateTags($tags);
diff --git a/core/modules/system/lib/Drupal/system/Tests/Cache/PageCacheTagsIntegrationTest.php b/core/modules/system/lib/Drupal/system/Tests/Cache/PageCacheTagsIntegrationTest.php
index 2dffc4f..1f0c39e 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Cache/PageCacheTagsIntegrationTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Cache/PageCacheTagsIntegrationTest.php
@@ -66,24 +66,24 @@ function testPageCacheTags() {
 
     // Full node page 1.
     $this->verifyPageCacheTags('node/' . $node_1->id(), array(
+      'content:1',
       'node_view:1',
       'node:' . $node_1->id(),
       'user:' . $author_1->id(),
       'filter_format:basic_html',
       'menu:footer',
       'menu:main',
-      'content:1',
     ));
 
     // Full node page 2.
     $this->verifyPageCacheTags('node/' . $node_2->id(), array(
+      'content:1',
       'node_view:1',
       'node:' . $node_2->id(),
       'user:' . $author_2->id(),
       'filter_format:full_html',
       'menu:footer',
       'menu:main',
-      'content:1',
     ));
   }
 
@@ -96,17 +96,22 @@ function testPageCacheTags() {
    *   The expected cache tags for the page cache entry of the given $path.
    */
   protected function verifyPageCacheTags($path, $expected_tags) {
-    $expected_tags_header = implode(' ', $expected_tags);
+    sort($expected_tags);
     $this->drupalGet($path);
     $this->assertEqual($this->drupalGetHeader('X-Drupal-Cache'), 'MISS');
-    $this->assertIdentical($this->drupalGetHeader('X-Drupal-Cache-Tags'), $expected_tags_header);
+    $actual_tags = explode(' ', $this->drupalGetHeader('X-Drupal-Cache-Tags'));
+    sort($actual_tags);
+    $this->assertIdentical($actual_tags, $expected_tags);
     $this->drupalGet($path);
+    $actual_tags = explode(' ', $this->drupalGetHeader('X-Drupal-Cache-Tags'));
+    sort($actual_tags);
     $this->assertEqual($this->drupalGetHeader('X-Drupal-Cache'), 'HIT');
-    $this->assertIdentical($this->drupalGetHeader('X-Drupal-Cache-Tags'), $expected_tags_header);
+    $this->assertIdentical($actual_tags, $expected_tags);
     $cid_parts = array(url($path, array('absolute' => TRUE)), 'html');
     $cid = sha1(implode(':', $cid_parts));
     $cache_entry = \Drupal::cache('page')->get($cid);
-    $this->assertIdentical($cache_entry->tags, $expected_tags);
+    sort($cache_entry->tags);
+    $this->assertEqual($cache_entry->tags, $expected_tags);
   }
 
 }