diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index ea1dd1e753b5..34e1ee516ca3 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -1,5 +1,5 @@
 <?php
-use Drupal\Core\Utility\CacheArray;
+
 /**
  * @file
  * The theme system, which controls the output of Drupal.
@@ -8,6 +8,8 @@ use Drupal\Core\Utility\CacheArray;
  * customized by user themes.
  */
 
+use Drupal\Core\Utility\ThemeRegistry;
+
 /**
  * @defgroup content_flags Content markers
  * @{
@@ -239,17 +241,17 @@ function _drupal_theme_initialize($theme, $base_theme = array(), $registry_callb
  *
  * @param bool $complete
  *   Optional boolean to indicate whether to return the complete theme registry
- *   array or an instance of the ThemeRegistry class. If TRUE, the complete
- *   theme registry array will be returned. This is useful if you want to
- *   foreach over the whole registry, use array_* functions or inspect it in a
- *   debugger. If FALSE, an instance of the ThemeRegistry class will be
- *   returned, this provides an ArrayObject which allows it to be accessed
- *   with array syntax and  isset(), and should be more lightweight
- *   than the full registry. Defaults to TRUE.
+ *   array or an instance of the Drupal\Core\Utility\ThemeRegistry class.
+ *   If TRUE, the complete theme registry array will be returned. This is useful
+ *   if you want to foreach over the whole registry, use array_* functions or
+ *   inspect it in a debugger. If FALSE, an instance of the
+ *   Drupal\Core\Utility\ThemeRegistry class will be returned, this provides an
+ *   ArrayObject which allows it to be accessed with array syntax and isset(),
+ *   and should be more lightweight than the full registry. Defaults to TRUE.
  *
  * @return
- *   The complete theme registry array, or an instance of the ThemeRegistry
- *   class.
+ *   The complete theme registry array, or an instance of the
+ *   Drupal\Core\Utility\ThemeRegistry class.
  */
 function theme_get_registry($complete = TRUE) {
   // Use the advanced drupal_static() pattern, since this is called very often.
@@ -307,10 +309,11 @@ function _theme_registry_callback($callback = NULL, array $arguments = array())
  *   The name of the theme engine.
  * @param $complete
  *   Whether to load the complete theme registry or an instance of the
- *   ThemeRegistry class.
+ *   Drupal\Core\Utility\ThemeRegistry class.
  *
  * @return
- *   The theme registry array, or an instance of the ThemeRegistry class.
+ *   The theme registry array, or an instance of the
+ *   Drupal\Core\Utility\ThemeRegistry class.
  */
 function _theme_load_registry($theme, $base_theme = NULL, $theme_engine = NULL, $complete = TRUE) {
   if ($complete) {
@@ -352,116 +355,6 @@ function drupal_theme_rebuild() {
   cache()->deletePrefix('theme_registry');
 }
 
-/**
- * Builds the run-time theme registry.
- *
- * Extends DrupalCacheArray to allow the theme registry to be accessed as a
- * complete registry, while internally caching only the parts of the registry
- * that are actually in use on the site. On cache misses the complete
- * theme registry is loaded and used to update the run-time cache.
- */
-class ThemeRegistry Extends CacheArray {
-
-  /**
-   * Whether the partial registry can be persisted to the cache.
-   *
-   * This is only allowed if all modules and the request method is GET. theme()
-   * should be very rarely called on POST requests and this avoids polluting
-   * the runtime cache.
-   */
-  protected $persistable;
-
-  /**
-   * The complete theme registry array.
-   */
-  protected $completeRegistry;
-
-  function __construct($cid, $bin) {
-    $this->cid = $cid;
-    $this->bin = $bin;
-    $this->persistable = module_load_all(NULL) && $_SERVER['REQUEST_METHOD'] == 'GET';
-
-    $data = array();
-    if ($this->persistable && $cached = cache($this->bin)->get($this->cid)) {
-      $data = $cached->data;
-    }
-    else {
-      // If there is no runtime cache stored, fetch the full theme registry,
-      // but then initialize each value to NULL. This allows offsetExists()
-      // to function correctly on non-registered theme hooks without triggering
-      // a call to resolveCacheMiss().
-      $data = $this->initializeRegistry();
-      if ($this->persistable) {
-        $this->set($data);
-      }
-    }
-    $this->storage = $data;
-  }
-
-  /**
-   * Initializes the full theme registry.
-   *
-   * @return
-   *   An array with the keys of the full theme registry, but the values
-   *   initialized to NULL.
-   */
-  function initializeRegistry() {
-    $this->completeRegistry = theme_get_registry();
-
-    return array_fill_keys(array_keys($this->completeRegistry), NULL);
-  }
-
-  public function offsetExists($offset) {
-    // Since the theme registry allows for theme hooks to be requested that
-    // are not registered, just check the existence of the key in the registry.
-    // Use array_key_exists() here since a NULL value indicates that the theme
-    // hook exists but has not yet been requested.
-    return array_key_exists($offset, $this->storage);
-  }
-
-  public function offsetGet($offset) {
-    // If the offset is set but empty, it is a registered theme hook that has
-    // not yet been requested. Offsets that do not exist at all were not
-    // registered in hook_theme().
-    if (isset($this->storage[$offset])) {
-      return $this->storage[$offset];
-    }
-    elseif (array_key_exists($offset, $this->storage)) {
-      return $this->resolveCacheMiss($offset);
-    }
-  }
-
-  public function resolveCacheMiss($offset) {
-    if (!isset($this->completeRegistry)) {
-      $this->completeRegistry = theme_get_registry();
-    }
-    $this->storage[$offset] = $this->completeRegistry[$offset];
-    if ($this->persistable) {
-      $this->persist($offset);
-    }
-    return $this->storage[$offset];
-  }
-
-  public function set($data, $lock = TRUE) {
-    $lock_name = $this->cid . ':' . $this->bin;
-    if (!$lock || lock_acquire($lock_name)) {
-      if ($cached = cache($this->bin)->get($this->cid)) {
-        // Use array merge instead of union so that filled in values in $data
-        // overwrite empty values in the current cache.
-        $data = array_merge($cached->data, $data);
-      }
-      else {
-        $registry = $this->initializeRegistry();
-        $data = array_merge($registry, $data);
-      }
-      cache($this->bin)->set($this->cid, $data);
-      if ($lock) {
-        lock_release($lock_name);
-      }
-    }
-  }
-}
-
 /**
  * Process a single implementation of hook_theme().
  *
diff --git a/core/lib/Drupal/Core/Utility/ThemeRegistry.php b/core/lib/Drupal/Core/Utility/ThemeRegistry.php
new file mode 100644
index 000000000000..e9512e9de8dd
--- /dev/null
+++ b/core/lib/Drupal/Core/Utility/ThemeRegistry.php
@@ -0,0 +1,118 @@
+<?php
+
+/**
+ * @file
+ * Definition of Drupal\Core\Utility\ThemeRegistry
+ */
+
+namespace Drupal\Core\Utility;
+
+/**
+ * Builds the run-time theme registry.
+ *
+ * Extends DrupalCacheArray to allow the theme registry to be accessed as a
+ * complete registry, while internally caching only the parts of the registry
+ * that are actually in use on the site. On cache misses the complete
+ * theme registry is loaded and used to update the run-time cache.
+ */
+class ThemeRegistry extends CacheArray {
+
+  /**
+   * Whether the partial registry can be persisted to the cache.
+   *
+   * This is only allowed if all modules and the request method is GET. theme()
+   * should be very rarely called on POST requests and this avoids polluting
+   * the runtime cache.
+   */
+  protected $persistable;
+
+  /**
+   * The complete theme registry array.
+   */
+  protected $completeRegistry;
+
+  function __construct($cid, $bin) {
+    $this->cid = $cid;
+    $this->bin = $bin;
+    $this->persistable = module_load_all(NULL) && $_SERVER['REQUEST_METHOD'] == 'GET';
+
+    $data = array();
+    if ($this->persistable && $cached = cache($this->bin)->get($this->cid)) {
+      $data = $cached->data;
+    }
+    else {
+      // If there is no runtime cache stored, fetch the full theme registry,
+      // but then initialize each value to NULL. This allows offsetExists()
+      // to function correctly on non-registered theme hooks without triggering
+      // a call to resolveCacheMiss().
+      $data = $this->initializeRegistry();
+      if ($this->persistable) {
+        $this->set($data);
+      }
+    }
+    $this->storage = $data;
+  }
+
+  /**
+   * Initializes the full theme registry.
+   *
+   * @return
+   *   An array with the keys of the full theme registry, but the values
+   *   initialized to NULL.
+   */
+  function initializeRegistry() {
+    $this->completeRegistry = theme_get_registry();
+
+    return array_fill_keys(array_keys($this->completeRegistry), NULL);
+  }
+
+  public function offsetExists($offset) {
+    // Since the theme registry allows for theme hooks to be requested that
+    // are not registered, just check the existence of the key in the registry.
+    // Use array_key_exists() here since a NULL value indicates that the theme
+    // hook exists but has not yet been requested.
+    return array_key_exists($offset, $this->storage);
+  }
+
+  public function offsetGet($offset) {
+    // If the offset is set but empty, it is a registered theme hook that has
+    // not yet been requested. Offsets that do not exist at all were not
+    // registered in hook_theme().
+    if (isset($this->storage[$offset])) {
+      return $this->storage[$offset];
+    }
+    elseif (array_key_exists($offset, $this->storage)) {
+      return $this->resolveCacheMiss($offset);
+    }
+  }
+
+  public function resolveCacheMiss($offset) {
+    if (!isset($this->completeRegistry)) {
+      $this->completeRegistry = theme_get_registry();
+    }
+    $this->storage[$offset] = $this->completeRegistry[$offset];
+    if ($this->persistable) {
+      $this->persist($offset);
+    }
+    return $this->storage[$offset];
+  }
+
+  public function set($data, $lock = TRUE) {
+    $lock_name = $this->cid . ':' . $this->bin;
+    if (!$lock || lock_acquire($lock_name)) {
+      if ($cached = cache($this->bin)->get($this->cid)) {
+        // Use array merge instead of union so that filled in values in $data
+        // overwrite empty values in the current cache.
+        $data = array_merge($cached->data, $data);
+      }
+      else {
+        $registry = $this->initializeRegistry();
+        $data = array_merge($registry, $data);
+      }
+      cache($this->bin)->set($this->cid, $data);
+      if ($lock) {
+        lock_release($lock_name);
+      }
+    }
+  }
+}
diff --git a/core/modules/system/lib/Drupal/system/Tests/Theme/RegistryTest.php b/core/modules/system/lib/Drupal/system/Tests/Theme/RegistryTest.php
index 41e0a694ef2b..16abc7852904 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Theme/RegistryTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/RegistryTest.php
@@ -8,7 +8,7 @@
 namespace Drupal\system\Tests\Theme;
 
 use Drupal\simpletest\WebTestBase;
-use ThemeRegistry;
+use Drupal\Core\Utility\ThemeRegistry;
 
 /**
  * Tests for the ThemeRegistry class.