diff --git a/core/tests/Drupal/Tests/Core/Entity/EntityTypeManagerTest.php b/core/tests/Drupal/Tests/Core/Entity/EntityTypeManagerTest.php
index 2e6c386..43464d1 100644
--- a/core/tests/Drupal/Tests/Core/Entity/EntityTypeManagerTest.php
+++ b/core/tests/Drupal/Tests/Core/Entity/EntityTypeManagerTest.php
@@ -97,8 +97,7 @@ protected function setUpEntityTypeDefinitions($definitions = []) {
       $entity_type->getLinkTemplates()->willReturn([]);
 
       // Give the entity type a legitimate class to return.
-      $entity_type->getClass()->willReturn('\\' . $class);
-      // Leading slashes will be trimmed.
+      $entity_type->getClass()->willReturn($class);
       $entity_type->setClass($class)->willReturn($entity_type->reveal());
 
       $definitions[$key] = $entity_type->reveal();
diff --git a/core/tests/Drupal/Tests/Core/Plugin/DefaultPluginManagerTest.php b/core/tests/Drupal/Tests/Core/Plugin/DefaultPluginManagerTest.php
index 4ced547..0a6e8d4 100644
--- a/core/tests/Drupal/Tests/Core/Plugin/DefaultPluginManagerTest.php
+++ b/core/tests/Drupal/Tests/Core/Plugin/DefaultPluginManagerTest.php
@@ -438,6 +438,17 @@ public function providerTestProcessDefinition() {
       'forms' => ['configure' => 'stdClass'],
       'foo' => ['bar' => ['baz']],
     ];
+
+    $data['class_with_slashes'][] = [
+      'class' => '\Drupal\Tests\Core\Plugin\TestPluginForm',
+    ];
+    $data['class_with_slashes'][] = [
+      'class' => 'Drupal\Tests\Core\Plugin\TestPluginForm',
+      'foo' => ['bar' => ['baz']],
+    ];
+
+    $data['object_with_class_with_slashes'][] = (new PluginDefinition())->setClass('\Drupal\Tests\Core\Plugin\TestPluginForm');
+    $data['object_with_class_with_slashes'][] = (new PluginDefinition())->setClass('Drupal\Tests\Core\Plugin\TestPluginForm');
     return $data;
   }