diff --git a/core/lib/Drupal/Component/Diff/DiffEngine.php b/core/lib/Drupal/Component/Diff/DiffEngine.php
index 0d1ec5c..ddbc79f 100644
--- a/core/lib/Drupal/Component/Diff/DiffEngine.php
+++ b/core/lib/Drupal/Component/Diff/DiffEngine.php
@@ -10,6 +10,8 @@
 
 define('USE_ASSERTS', FALSE);
 
+use Drupal\Component\Utility\String;
+
 /**
  * @todo document
  * @private
@@ -960,10 +962,10 @@ function _HWLDF_WordAccumulator() {
   function _flushGroup($new_tag) {
     if ($this->_group !== '') {
       if ($this->_tag == 'mark') {
-        $this->_line .= '<span class="diffchange">' . check_plain($this->_group) . '</span>';
+        $this->_line .= '<span class="diffchange">' . String::checkPlain($this->_group) . '</span>';
       }
       else {
-        $this->_line .= check_plain($this->_group);
+        $this->_line .= String::checkPlain($this->_group);
       }
     }
     $this->_group = '';
@@ -1187,19 +1189,19 @@ function emptyLine() {
 
   function _added($lines) {
     foreach ($lines as $line) {
-      $this->rows[] = array_merge($this->emptyLine(), $this->addedLine(check_plain($line)));
+      $this->rows[] = array_merge($this->emptyLine(), $this->addedLine(String::checkPlain($line)));
     }
   }
 
   function _deleted($lines) {
     foreach ($lines as $line) {
-      $this->rows[] = array_merge($this->deletedLine(check_plain($line)), $this->emptyLine());
+      $this->rows[] = array_merge($this->deletedLine(String::checkPlain($line)), $this->emptyLine());
     }
   }
 
   function _context($lines) {
     foreach ($lines as $line) {
-      $this->rows[] = array_merge($this->contextLine(check_plain($line)), $this->contextLine(check_plain($line)));
+      $this->rows[] = array_merge($this->contextLine(String::checkPlain($line)), $this->contextLine(String::checkPlain($line)));
     }
   }
 
diff --git a/core/lib/Drupal/Component/Utility/Xss.php b/core/lib/Drupal/Component/Utility/Xss.php
index 8d0d257..3bf3d87 100644
--- a/core/lib/Drupal/Component/Utility/Xss.php
+++ b/core/lib/Drupal/Component/Utility/Xss.php
@@ -88,7 +88,8 @@ public static function filter($string, $allowed_tags = array('a', 'em', 'strong'
    *
    * Use only for fields where it is impractical to use the
    * whole filter system, but where some (mainly inline) mark-up
-   * is desired (so check_plain() is not acceptable).
+   * is desired (so \Drupal\Component\Utility\String::checkPlain() is not
+   * acceptable).
    *
    * Allows all tags that can be used inside an HTML body, save
    * for scripts and styles.
diff --git a/core/lib/Drupal/Core/Controller/ExceptionController.php b/core/lib/Drupal/Core/Controller/ExceptionController.php
index b03cc8e..f407ca7 100644
--- a/core/lib/Drupal/Core/Controller/ExceptionController.php
+++ b/core/lib/Drupal/Core/Controller/ExceptionController.php
@@ -13,6 +13,7 @@
 use Symfony\Component\HttpFoundation\JsonResponse;
 use Symfony\Component\HttpKernel\HttpKernelInterface;
 use Symfony\Component\HttpKernel\Exception\FlattenException;
+use Drupal\Component\Utility\String;
 
 use Drupal\Core\ContentNegotiation;
 
@@ -136,7 +137,7 @@ public function on403Html(FlattenException $exception, Request $request) {
    *   The request object that triggered this exception.
    */
   public function on404Html(FlattenException $exception, Request $request) {
-    watchdog('page not found', check_plain($request->attributes->get('_system_path')), NULL, WATCHDOG_WARNING);
+    watchdog('page not found', String::checkPlain($request->attributes->get('_system_path')), NULL, WATCHDOG_WARNING);
 
     // Check for and return a fast 404 page if configured.
     $config = \Drupal::config('system.performance');
@@ -146,7 +147,7 @@ public function on404Html(FlattenException $exception, Request $request) {
       $fast_paths = $config->get('fast_404.paths');
       if ($fast_paths && preg_match($fast_paths, $request->getPathInfo())) {
         $fast_404_html = $config->get('fast_404.html');
-        $fast_404_html = strtr($fast_404_html, array('@path' => check_plain($request->getUri())));
+        $fast_404_html = strtr($fast_404_html, array('@path' => String::checkPlain($request->getUri())));
         return new Response($fast_404_html, 404);
       }
     }
@@ -353,7 +354,7 @@ protected function decodeException(FlattenException $exception) {
       '%type' => $exception->getClass(),
       // The standard PHP exception handler considers that the exception message
       // is plain-text. We mimick this behavior here.
-      '!message' => check_plain($message),
+      '!message' => String::checkPlain($message),
       '%function' => $caller['function'],
       '%file' => $caller['file'],
       '%line' => $caller['line'],
diff --git a/core/lib/Drupal/Core/Entity/EntityNG.php b/core/lib/Drupal/Core/Entity/EntityNG.php
index 35e002a..f6ca10b 100644
--- a/core/lib/Drupal/Core/Entity/EntityNG.php
+++ b/core/lib/Drupal/Core/Entity/EntityNG.php
@@ -7,6 +7,7 @@
 
 namespace Drupal\Core\Entity;
 
+use Drupal\Component\Utility\String;
 use Drupal\Core\Language\Language;
 use Drupal\Core\Session\AccountInterface;
 use Drupal\Core\TypedData\TypedDataInterface;
@@ -287,7 +288,7 @@ protected function getTranslatedField($property_name, $langcode) {
     if (!isset($this->fields[$property_name][$langcode])) {
       $definition = $this->getPropertyDefinition($property_name);
       if (!$definition) {
-        throw new \InvalidArgumentException('Field ' . check_plain($property_name) . ' is unknown.');
+        throw new \InvalidArgumentException('Field ' . String::checkPlain($property_name) . ' is unknown.');
       }
       // Non-translatable fields are always stored with
       // Language::LANGCODE_DEFAULT as key.
diff --git a/core/lib/Drupal/Core/Session/AccountInterface.php b/core/lib/Drupal/Core/Session/AccountInterface.php
index fc1ab15..ecb158e 100644
--- a/core/lib/Drupal/Core/Session/AccountInterface.php
+++ b/core/lib/Drupal/Core/Session/AccountInterface.php
@@ -120,8 +120,9 @@ public function getPreferredAdminLangcode($default = NULL);
    *
    * @return
    *   An unsanitized string with the username to display. The code receiving
-   *   this result must ensure that check_plain() is called on it before it is
-   *   printed to the page.
+   *   this result must ensure that
+   *   \Drupal\Component\Utility\String::checkPlain() is called on it before it
+   *   is printed to the page.
    */
   public function getUsername();
 
diff --git a/core/lib/Drupal/Core/TypedData/TypedDataManager.php b/core/lib/Drupal/Core/TypedData/TypedDataManager.php
index bbc0d7f..6605f39 100644
--- a/core/lib/Drupal/Core/TypedData/TypedDataManager.php
+++ b/core/lib/Drupal/Core/TypedData/TypedDataManager.php
@@ -8,6 +8,7 @@
 namespace Drupal\Core\TypedData;
 
 use Drupal\Component\Plugin\Exception\PluginException;
+use Drupal\Component\Utility\String;
 use Drupal\Core\Cache\CacheBackendInterface;
 use Drupal\Core\Extension\ModuleHandlerInterface;
 use Drupal\Core\Language\LanguageManager;
@@ -239,7 +240,7 @@ public function getPropertyInstance(TypedDataInterface $object, $property_name,
       }
       // Make sure we have got a valid definition.
       if (!$definition) {
-        throw new \InvalidArgumentException('Property ' . check_plain($property_name) . ' is unknown.');
+        throw new \InvalidArgumentException('Property ' . String::checkPlain($property_name) . ' is unknown.');
       }
       // Now create the prototype using the definition, but do not pass the
       // given value as it will serve as prototype for any further instance.
diff --git a/core/lib/Drupal/Core/Utility/Token.php b/core/lib/Drupal/Core/Utility/Token.php
index 29fef78..b2c38dd 100644
--- a/core/lib/Drupal/Core/Utility/Token.php
+++ b/core/lib/Drupal/Core/Utility/Token.php
@@ -108,8 +108,8 @@ public function __construct(ModuleHandlerInterface $module_handler) {
    *   - sanitize: A boolean flag indicating that tokens should be sanitized for
    *     display to a web browser. Defaults to TRUE. Developers who set this
    *     option to FALSE assume responsibility for running filter_xss(),
-   *     check_plain() or other appropriate scrubbing functions before displaying
-   *     data to users.
+   *     \Drupal\Component\Utility\String::checkPlain() or other appropriate
+   *     scrubbing functions before displaying data to users.
    *
    * @return string
    *   Text with tokens replaced.
@@ -200,8 +200,9 @@ public function scan($text) {
    *     encoding or truncation to a specific length.
    *   - sanitize: A boolean flag indicating that tokens should be sanitized for
    *     display to a web browser. Developers who set this option to FALSE assume
-   *     responsibility for running filter_xss(), check_plain() or other
-   *     appropriate scrubbing functions before displaying data to users.
+   *     responsibility for running filter_xss(),
+   *     \Drupal\Component\Utility\String::checkPlain() or other appropriate
+   *     scrubbing functions before displaying data to users.
    *
    * @return array
    *   An associative array of replacement values, keyed by the original 'raw'