Change 'cache_key' to AbstractRendererEngine::CACHE_KEY_VAR

The purpose of this change is to find all usages of AbstractRendererEngine::CACHE_KEY_VAR. Currently, if you search for AbstractRendererEngine::CACHE_KEY_VAR you will see only access to it, i.e. (`$view->vars[AbstractRendererEngine::CACHE_KEY_VAR]`), but you can't find it in write level. With this pull request you can see where is was used for write.

Author: vv12131415

Extension/Core/Type/BaseType.php

@@ -11,6 +11,7 @@
 
 namespace Symfony\Component\Form\Extension\Core\Type;
 
+use Symfony\Component\Form\AbstractRendererEngine;
 use Symfony\Component\Form\AbstractType;
 use Symfony\Component\Form\FormBuilderInterface;
 use Symfony\Component\Form\FormInterface;
@@ -100,7 +101,7 @@ public function buildView(FormView $view, FormInterface $form, array $options)
             // collection form have different types (dynamically), they should
             // be rendered differently.
             // https://github.com/symfony/symfony/issues/5038
-            'cache_key' => $uniqueBlockPrefix.'_'.$form->getConfig()->getType()->getBlockPrefix(),
+            AbstractRendererEngine::CACHE_KEY_VAR => $uniqueBlockPrefix.'_'.$form->getConfig()->getType()->getBlockPrefix(),
         ]);
     }