minor #18025 [2.7] Don't use reflection when possible (Ener-Getick)

This PR was merged into the 2.7 branch.

Discussion
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[2.7] Don't use reflection when possible

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |

See symfony/symfony#18021

Commits
-------

35be501 Don't use reflections when possible

Author: fabpot

DependencyInjection/FragmentRendererPass.php

@@ -52,9 +52,9 @@ public function process(ContainerBuilder $container)
                 throw new \InvalidArgumentException(sprintf('The service "%s" must not be abstract as fragment renderer are lazy-loaded.', $id));
             }
 
-            $refClass = new \ReflectionClass($container->getParameterBag()->resolveValue($def->getClass()));
+            $class = $container->getParameterBag()->resolveValue($def->getClass());
             $interface = 'Symfony\Component\HttpKernel\Fragment\FragmentRendererInterface';
-            if (!$refClass->implementsInterface($interface)) {
+            if (!is_subclass_of($class, $interface)) {
                 throw new \InvalidArgumentException(sprintf('Service "%s" must implement interface "%s".', $id, $interface));
             }