diff --git a/core/modules/simpletest/src/AssertHelperTrait.php b/core/modules/simpletest/src/AssertHelperTrait.php
index 5325e8c9b0..2d1d6f2900 100644
--- a/core/modules/simpletest/src/AssertHelperTrait.php
+++ b/core/modules/simpletest/src/AssertHelperTrait.php
@@ -2,34 +2,18 @@
 
 namespace Drupal\simpletest;
 
-use Drupal\Component\Render\MarkupInterface;
+use Drupal\Tests\AssertHelperTrait as BaseAssertHelperTrait;
+
+@trigger_error(__NAMESPACE__ . '\AssertHelperTrait is deprecated in Drupal 8.4.x and will be removed before Drupal 9.0.0. Instead, use Drupal\Tests\AssertHelperTrait.', E_USER_DEPRECATED);
 
 /**
  * Provides helper methods for assertions.
+ *
+ * @deprecated in Drupal 8.4.x. Will be removed before Drupal 9.0.0. Use
+ *   Drupal\Tests\AssertHelperTrait instead.
  */
 trait AssertHelperTrait {
 
-  /**
-   * Casts MarkupInterface objects into strings.
-   *
-   * @param string|array $value
-   *   The value to act on.
-   *
-   * @return mixed
-   *   The input value, with MarkupInterface objects casted to string.
-   */
-  protected static function castSafeStrings($value) {
-    if ($value instanceof MarkupInterface) {
-      $value = (string) $value;
-    }
-    if (is_array($value)) {
-      array_walk_recursive($value, function (&$item) {
-        if ($item instanceof MarkupInterface) {
-          $item = (string) $item;
-        }
-      });
-    }
-    return $value;
-  }
+  use BaseAssertHelperTrait;
 
 }
diff --git a/core/modules/simpletest/src/AssertHelperTrait.php b/core/tests/Drupal/Tests/AssertHelperTrait.php
similarity index 96%
copy from core/modules/simpletest/src/AssertHelperTrait.php
copy to core/tests/Drupal/Tests/AssertHelperTrait.php
index 5325e8c9b0..ba814d9641 100644
--- a/core/modules/simpletest/src/AssertHelperTrait.php
+++ b/core/tests/Drupal/Tests/AssertHelperTrait.php
@@ -1,6 +1,6 @@
 <?php
 
-namespace Drupal\simpletest;
+namespace Drupal\Tests;
 
 use Drupal\Component\Render\MarkupInterface;
 
diff --git a/core/modules/simpletest/tests/src/Unit/AssertHelperTraitTest.php b/core/tests/Drupal/Tests/AssertHelperTraitTest.php
similarity index 82%
rename from core/modules/simpletest/tests/src/Unit/AssertHelperTraitTest.php
rename to core/tests/Drupal/Tests/AssertHelperTraitTest.php
index 8c816b0bbe..81d6d7a448 100644
--- a/core/modules/simpletest/tests/src/Unit/AssertHelperTraitTest.php
+++ b/core/tests/Drupal/Tests/AssertHelperTraitTest.php
@@ -1,19 +1,15 @@
 <?php
 
-/**
- * @file
- * Contains \Drupal\Tests\simpletest\Unit\AssertHelperTraitTest.
- */
-
-namespace Drupal\Tests\simpletest\Unit;
+namespace Drupal\Tests;
 
 use Drupal\Core\Render\Markup;
-use Drupal\simpletest\AssertHelperTrait;
+use Drupal\Tests\AssertHelperTrait;
 use Drupal\Tests\UnitTestCase;
 
 /**
- * @coversDefaultClass \Drupal\simpletest\AssertHelperTrait
+ * @coversDefaultClass \Drupal\Tests\AssertHelperTrait
  * @group simpletest
+ * @group Tests
  */
 class AssertHelperTraitTest extends UnitTestCase {