Add a class_exists check.

Author: gnutix

src/Test/WebTestCase.php

@@ -33,9 +33,6 @@
 use Symfony\Component\Security\Core\Authentication\Token\UsernamePasswordToken;
 use Symfony\Component\Security\Core\User\UserInterface;
 
-// Compatibility layer for Symfony 4.3+
-class_alias('Symfony\Bundle\FrameworkBundle\KernelBrowser', 'Symfony\Bundle\FrameworkBundle\Client');
-
 /**
  * @author Lea Haensenberger
  * @author Lukas Kahwe Smith <smith@pooteeweet.org>
@@ -571,3 +568,8 @@ protected function tearDown(): void
         parent::tearDown();
     }
 }
+
+// Compatibility layer for Symfony 4.3+
+if (class_exists('Symfony\Bundle\FrameworkBundle\KernelBrowser')) {
+    class_alias('Symfony\Bundle\FrameworkBundle\KernelBrowser', 'Symfony\Bundle\FrameworkBundle\Client');
+}