diff --git a/core/modules/simpletest/simpletest.module b/core/modules/simpletest/simpletest.module
index 8a78058..1d291c7 100644
--- a/core/modules/simpletest/simpletest.module
+++ b/core/modules/simpletest/simpletest.module
@@ -645,6 +645,10 @@ function simpletest_generate_file($filename, $width, $lines, $type = 'binary-tex
  * Removes all temporary database tables and directories.
  */
 function simpletest_clean_environment() {
+  // We have to register the test namespaces so that core's test classes are
+  // accessible to these functions.
+  // @todo: Remove/refactor in https://www.drupal.org/node/2800267
+  \Drupal::service('test_discovery')->registerTestNamespaces();
   simpletest_clean_database();
   simpletest_clean_temporary_directories();
   if (\Drupal::config('simpletest.settings')->get('clear_results')) {
@@ -694,7 +698,7 @@ function simpletest_clean_temporary_directories() {
     foreach ($files as $file) {
       if ($file[0] != '.') {
         $path = DRUPAL_ROOT . '/sites/simpletest/' . $file;
-        file_unmanaged_delete_recursive($path, 'simpletest_temp_directory_pre_delete_callback');
+        file_unmanaged_delete_recursive($path, array('Drupal\simpletest\TestBase', 'filePreDeleteCallback'));
         $count++;
       }
     }
@@ -860,17 +864,3 @@ function simpletest_phpunit_testcase_to_row($test_id, \SimpleXMLElement $testcas
   );
   return $record;
 }
-
-/**
- * Ensures test files are deletable within file_unmanaged_delete_recursive().
- *
- * Some tests chmod generated files to be read only. During
- * simpletest_clean_environment() and other cleanup operations, these files
- * need to get deleted too.
- *
- * @param string $path
- *   The path to be deleted.
- */
-function simpletest_temp_directory_pre_delete_callback($path) {
-  chmod($path, 0700);
-}
diff --git a/core/modules/simpletest/src/TestBase.php b/core/modules/simpletest/src/TestBase.php
index b3f8ee0..039ddfc 100644
--- a/core/modules/simpletest/src/TestBase.php
+++ b/core/modules/simpletest/src/TestBase.php
@@ -1509,9 +1509,14 @@ public static function generatePermutations($parameters) {
    * Some tests chmod generated files to be read only. During
    * TestBase::restoreEnvironment() and other cleanup operations, these files
    * need to get deleted too.
+   *
+   * @param string $path
+   *   The path about to be deleted.
+   *
+   * @todo Remove/refactor in https://www.drupal.org/node/2800267
    */
   public static function filePreDeleteCallback($path) {
-    simpletest_temp_directory_pre_delete_callback($path);
+    chmod($path, 0700);
   }
 
   /**