diff --git a/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
index f71e1a2..60a0d5d 100644
--- a/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
@@ -10,6 +10,7 @@
 use Drupal\Component\Utility\Random;
 use Drupal\Core\Database\Database;
 use Drupal\Component\Utility\Settings;
+use Drupal\Component\Utility\String;
 use Drupal\Core\Config\ConfigImporter;
 use Drupal\Core\Config\StorageComparer;
 use Drupal\Core\DependencyInjection\ContainerBuilder;
@@ -240,8 +241,9 @@ protected function checkRequirements() {
    *   TRUE is a synonym for 'pass', FALSE for 'fail'.
    * @param $message
    *   (optional) A message to display with the assertion. Do not translate
-   *   messages: use format_string() to embed variables in the message text, not
-   *   t(). If left blank, a default message will be displayed.
+   *   messages: use \Drupal\Component\Utility\String::format() to embed
+   *   variables in the message text, not t(). If left blank, a default message
+   *   will be displayed.
    * @param $group
    *   (optional) The group this message is in, which is displayed in a column
    *   in test output. Use 'Debug' to indicate this is debugging output. Do not
@@ -322,9 +324,9 @@ public static function insertAssert($test_id, $test_class, $status, $message = '
     }
 
     $caller += array(
-      'function' => t('Unknown'),
+      'function' => 'Unknown',
       'line' => 0,
-      'file' => t('Unknown'),
+      'file' => 'Unknown',
     );
 
     $assertion = array(
@@ -411,8 +413,9 @@ protected function getAssertionCall() {
    *   The value on which the assertion is to be done.
    * @param $message
    *   (optional) A message to display with the assertion. Do not translate
-   *   messages: use format_string() to embed variables in the message text, not
-   *   t(). If left blank, a default message will be displayed.
+   *   messages: use \Drupal\Component\Utility\String::format() to embed
+   *   variables in the message text, not t(). If left blank, a default message
+   *   will be displayed.
    * @param $group
    *   (optional) The group this message is in, which is displayed in a column
    *   in test output. Use 'Debug' to indicate this is debugging output. Do not
@@ -423,7 +426,7 @@ protected function getAssertionCall() {
    *   TRUE if the assertion succeeded, FALSE otherwise.
    */
   protected function assertTrue($value, $message = '', $group = 'Other') {
-    return $this->assert((bool) $value, $message ? $message : t('Value @value is TRUE.', array('@value' => var_export($value, TRUE))), $group);
+    return $this->assert((bool) $value, $message ? $message : String::format('Value @value is TRUE.', array('@value' => var_export($value, TRUE))), $group);
   }
 
   /**
@@ -435,8 +438,9 @@ protected function assertTrue($value, $message = '', $group = 'Other') {
    *   The value on which the assertion is to be done.
    * @param $message
    *   (optional) A message to display with the assertion. Do not translate
-   *   messages: use format_string() to embed variables in the message text, not
-   *   t(). If left blank, a default message will be displayed.
+   *   messages: use \Drupal\Component\Utility\String::format() to embed
+   *   variables in the message text, not t(). If left blank, a default message
+   *   will be displayed.
    * @param $group
    *   (optional) The group this message is in, which is displayed in a column
    *   in test output. Use 'Debug' to indicate this is debugging output. Do not
@@ -447,7 +451,7 @@ protected function assertTrue($value, $message = '', $group = 'Other') {
    *   TRUE if the assertion succeeded, FALSE otherwise.
    */
   protected function assertFalse($value, $message = '', $group = 'Other') {
-    return $this->assert(!$value, $message ? $message : t('Value @value is FALSE.', array('@value' => var_export($value, TRUE))), $group);
+    return $this->assert(!$value, $message ? $message : String::format('Value @value is FALSE.', array('@value' => var_export($value, TRUE))), $group);
   }
 
   /**
@@ -457,8 +461,9 @@ protected function assertFalse($value, $message = '', $group = 'Other') {
    *   The value on which the assertion is to be done.
    * @param $message
    *   (optional) A message to display with the assertion. Do not translate
-   *   messages: use format_string() to embed variables in the message text, not
-   *   t(). If left blank, a default message will be displayed.
+   *   messages: use \Drupal\Component\Utility\String::format() to embed
+   *   variables in the message text, not t(). If left blank, a default message
+   *   will be displayed.
    * @param $group
    *   (optional) The group this message is in, which is displayed in a column
    *   in test output. Use 'Debug' to indicate this is debugging output. Do not
@@ -469,7 +474,7 @@ protected function assertFalse($value, $message = '', $group = 'Other') {
    *   TRUE if the assertion succeeded, FALSE otherwise.
    */
   protected function assertNull($value, $message = '', $group = 'Other') {
-    return $this->assert(!isset($value), $message ? $message : t('Value @value is NULL.', array('@value' => var_export($value, TRUE))), $group);
+    return $this->assert(!isset($value), $message ? $message : String::format('Value @value is NULL.', array('@value' => var_export($value, TRUE))), $group);
   }
 
   /**
@@ -479,8 +484,9 @@ protected function assertNull($value, $message = '', $group = 'Other') {
    *   The value on which the assertion is to be done.
    * @param $message
    *   (optional) A message to display with the assertion. Do not translate
-   *   messages: use format_string() to embed variables in the message text, not
-   *   t(). If left blank, a default message will be displayed.
+   *   messages: use \Drupal\Component\Utility\String::format() to embed
+   *   variables in the message text, not t(). If left blank, a default message
+   *   will be displayed.
    * @param $group
    *   (optional) The group this message is in, which is displayed in a column
    *   in test output. Use 'Debug' to indicate this is debugging output. Do not
@@ -491,7 +497,7 @@ protected function assertNull($value, $message = '', $group = 'Other') {
    *   TRUE if the assertion succeeded, FALSE otherwise.
    */
   protected function assertNotNull($value, $message = '', $group = 'Other') {
-    return $this->assert(isset($value), $message ? $message : t('Value @value is not NULL.', array('@value' => var_export($value, TRUE))), $group);
+    return $this->assert(isset($value), $message ? $message : String::format('Value @value is not NULL.', array('@value' => var_export($value, TRUE))), $group);
   }
 
   /**
@@ -503,8 +509,9 @@ protected function assertNotNull($value, $message = '', $group = 'Other') {
    *   The second value to check.
    * @param $message
    *   (optional) A message to display with the assertion. Do not translate
-   *   messages: use format_string() to embed variables in the message text, not
-   *   t(). If left blank, a default message will be displayed.
+   *   messages: use \Drupal\Component\Utility\String::format() to embed
+   *   variables in the message text, not t(). If left blank, a default message
+   *   will be displayed.
    * @param $group
    *   (optional) The group this message is in, which is displayed in a column
    *   in test output. Use 'Debug' to indicate this is debugging output. Do not
@@ -515,7 +522,7 @@ protected function assertNotNull($value, $message = '', $group = 'Other') {
    *   TRUE if the assertion succeeded, FALSE otherwise.
    */
   protected function assertEqual($first, $second, $message = '', $group = 'Other') {
-    return $this->assert($first == $second, $message ? $message : t('Value @first is equal to value @second.', array('@first' => var_export($first, TRUE), '@second' => var_export($second, TRUE))), $group);
+    return $this->assert($first == $second, $message ? $message : String::format('Value @first is equal to value @second.', array('@first' => var_export($first, TRUE), '@second' => var_export($second, TRUE))), $group);
   }
 
   /**
@@ -527,8 +534,9 @@ protected function assertEqual($first, $second, $message = '', $group = 'Other')
    *   The second value to check.
    * @param $message
    *   (optional) A message to display with the assertion. Do not translate
-   *   messages: use format_string() to embed variables in the message text, not
-   *   t(). If left blank, a default message will be displayed.
+   *   messages: use \Drupal\Component\Utility\String::format() to embed
+   *   variables in the message text, not t(). If left blank, a default message
+   *   will be displayed.
    * @param $group
    *   (optional) The group this message is in, which is displayed in a column
    *   in test output. Use 'Debug' to indicate this is debugging output. Do not
@@ -539,7 +547,7 @@ protected function assertEqual($first, $second, $message = '', $group = 'Other')
    *   TRUE if the assertion succeeded, FALSE otherwise.
    */
   protected function assertNotEqual($first, $second, $message = '', $group = 'Other') {
-    return $this->assert($first != $second, $message ? $message : t('Value @first is not equal to value @second.', array('@first' => var_export($first, TRUE), '@second' => var_export($second, TRUE))), $group);
+    return $this->assert($first != $second, $message ? $message : String::format('Value @first is not equal to value @second.', array('@first' => var_export($first, TRUE), '@second' => var_export($second, TRUE))), $group);
   }
 
   /**
@@ -551,8 +559,9 @@ protected function assertNotEqual($first, $second, $message = '', $group = 'Othe
    *   The second value to check.
    * @param $message
    *   (optional) A message to display with the assertion. Do not translate
-   *   messages: use format_string() to embed variables in the message text, not
-   *   t(). If left blank, a default message will be displayed.
+   *   messages: use \Drupal\Component\Utility\String::format() to embed
+   *   variables in the message text, not t(). If left blank, a default message
+   *   will be displayed.
    * @param $group
    *   (optional) The group this message is in, which is displayed in a column
    *   in test output. Use 'Debug' to indicate this is debugging output. Do not
@@ -563,7 +572,7 @@ protected function assertNotEqual($first, $second, $message = '', $group = 'Othe
    *   TRUE if the assertion succeeded, FALSE otherwise.
    */
   protected function assertIdentical($first, $second, $message = '', $group = 'Other') {
-    return $this->assert($first === $second, $message ? $message : t('Value @first is identical to value @second.', array('@first' => var_export($first, TRUE), '@second' => var_export($second, TRUE))), $group);
+    return $this->assert($first === $second, $message ? $message : String::format('Value @first is identical to value @second.', array('@first' => var_export($first, TRUE), '@second' => var_export($second, TRUE))), $group);
   }
 
   /**
@@ -575,8 +584,9 @@ protected function assertIdentical($first, $second, $message = '', $group = 'Oth
    *   The second value to check.
    * @param $message
    *   (optional) A message to display with the assertion. Do not translate
-   *   messages: use format_string() to embed variables in the message text, not
-   *   t(). If left blank, a default message will be displayed.
+   *   messages: use \Drupal\Component\Utility\String::format() to embed
+   *   variables in the message text, not t(). If left blank, a default message
+   *   will be displayed.
    * @param $group
    *   (optional) The group this message is in, which is displayed in a column
    *   in test output. Use 'Debug' to indicate this is debugging output. Do not
@@ -587,7 +597,7 @@ protected function assertIdentical($first, $second, $message = '', $group = 'Oth
    *   TRUE if the assertion succeeded, FALSE otherwise.
    */
   protected function assertNotIdentical($first, $second, $message = '', $group = 'Other') {
-    return $this->assert($first !== $second, $message ? $message : t('Value @first is not identical to value @second.', array('@first' => var_export($first, TRUE), '@second' => var_export($second, TRUE))), $group);
+    return $this->assert($first !== $second, $message ? $message : String::format('Value @first is not identical to value @second.', array('@first' => var_export($first, TRUE), '@second' => var_export($second, TRUE))), $group);
   }
 
   /**
@@ -599,8 +609,9 @@ protected function assertNotIdentical($first, $second, $message = '', $group = '
    *   The second object to check.
    * @param $message
    *   (optional) A message to display with the assertion. Do not translate
-   *   messages: use format_string() to embed variables in the message text, not
-   *   t(). If left blank, a default message will be displayed.
+   *   messages: use \Drupal\Component\Utility\String::format() to embed
+   *   variables in the message text, not t(). If left blank, a default message
+   *   will be displayed.
    * @param $group
    *   (optional) The group this message is in, which is displayed in a column
    *   in test output. Use 'Debug' to indicate this is debugging output. Do not
@@ -611,7 +622,7 @@ protected function assertNotIdentical($first, $second, $message = '', $group = '
    *   TRUE if the assertion succeeded, FALSE otherwise.
    */
   protected function assertIdenticalObject($object1, $object2, $message = '', $group = 'Other') {
-    $message = $message ?: format_string('!object1 is identical to !object2', array(
+    $message = $message ?: String::format('!object1 is identical to !object2', array(
       '!object1' => var_export($object1, TRUE),
       '!object2' => var_export($object2, TRUE),
     ));
@@ -642,8 +653,9 @@ protected function assertNoErrorsLogged() {
    *
    * @param $message
    *   (optional) A message to display with the assertion. Do not translate
-   *   messages: use format_string() to embed variables in the message text, not
-   *   t(). If left blank, a default message will be displayed.
+   *   messages: use \Drupal\Component\Utility\String::format() to embed
+   *   variables in the message text, not t(). If left blank, a default message
+   *   will be displayed.
    * @param $group
    *   (optional) The group this message is in, which is displayed in a column
    *   in test output. Use 'Debug' to indicate this is debugging output. Do not
@@ -662,8 +674,9 @@ protected function pass($message = NULL, $group = 'Other') {
    *
    * @param $message
    *   (optional) A message to display with the assertion. Do not translate
-   *   messages: use format_string() to embed variables in the message text, not
-   *   t(). If left blank, a default message will be displayed.
+   *   messages: use \Drupal\Component\Utility\String::format() to embed
+   *   variables in the message text, not t(). If left blank, a default message
+   *   will be displayed.
    * @param $group
    *   (optional) The group this message is in, which is displayed in a column
    *   in test output. Use 'Debug' to indicate this is debugging output. Do not
@@ -682,8 +695,9 @@ protected function fail($message = NULL, $group = 'Other') {
    *
    * @param $message
    *   (optional) A message to display with the assertion. Do not translate
-   *   messages: use format_string() to embed variables in the message text, not
-   *   t(). If left blank, a default message will be displayed.
+   *   messages: use \Drupal\Component\Utility\String::format() to embed
+   *   variables in the message text, not t(). If left blank, a default message
+   *   will be displayed.
    * @param $group
    *   (optional) The group this message is in, which is displayed in a column
    *   in test output. Use 'Debug' to indicate this is debugging output. Do not
@@ -728,7 +742,7 @@ protected function verbose($message) {
       $url = $this->verboseDirectoryUrl . '/' . $this->verboseClassName . '-' . $this->verboseId . '.html';
       // Not using l() to avoid invoking the theme system, so that unit tests
       // can use verbose() as well.
-      $url = '<a href="' . $url . '" target="_blank">' . t('Verbose message') . '</a>';
+      $url = '<a href="' . $url . '" target="_blank">Verbose message</a>';
       $this->error($url, 'User notice');
     }
     $this->verboseId++;
@@ -806,7 +820,7 @@ public function run(array $methods = array()) {
             'line' => $method_info->getStartLine(),
             'function' => $class . '->' . $method . '()',
           );
-          $completion_check_id = TestBase::insertAssert($this->testId, $class, FALSE, t('The test did not complete due to a fatal error.'), 'Completion check', $caller);
+          $completion_check_id = TestBase::insertAssert($this->testId, $class, FALSE, 'The test did not complete due to a fatal error.', 'Completion check', $caller);
           try {
             $this->prepareEnvironment();
           }
@@ -1143,8 +1157,8 @@ private function restoreEnvironment() {
       $captured_emails = $state->get('system.test_mail_collector') ?: array();
       $emailCount = count($captured_emails);
       if ($emailCount) {
-        $message = format_plural($emailCount, '1 e-mail was sent during this test.', '@count e-mails were sent during this test.');
-        $this->pass($message, t('E-mail'));
+        $message = $emailCount == 1 ? '1 e-mail was sent during this test.' : '@count e-mails were sent during this test.';
+        $this->pass($message, 'E-mail');
       }
     }
 
@@ -1271,7 +1285,7 @@ protected function exceptionHandler($exception) {
     // message through \Drupal\Component\Utility\String::checkPlain().
     $decoded_exception = Error::decodeException($exception);
     unset($decoded_exception['backtrace']);
-    $message = format_string('%type: !message in %function (line %line of %file). <pre class="backtrace">!backtrace</pre>', $decoded_exception + array(
+    $message = String::format('%type: !message in %function (line %line of %file). <pre class="backtrace">!backtrace</pre>', $decoded_exception + array(
       '!backtrace' => Error::formatBacktrace($verbose_backtrace),
     ));
     $this->error($message, 'Uncaught exception', Error::getLastCaller($backtrace));