bug symfony#24519 [Validator] [Twig] added magic method __isset() to File Constraint class (loru88)

This PR was submitted for the 3.4 branch but it was merged into the 2.7 branch instead (closes symfony#24519).

Discussion
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[Validator] [Twig] added magic method __isset()  to File Constraint class

| Q             | A
| ------------- | ---
| Branch?       | 3.4 or master / 2.7, 2.8 or 3.3 <!-- see comment below -->
| Bug fix?      | no
| New feature?  | yes <!-- don't forget to update src/**/CHANGELOG.md files -->
| BC breaks?    | no
| Deprecations? | no <!-- don't forget to update UPGRADE-*.md files -->
| Tests pass?   | yes
| Fixed tickets | symfony#24512  <!-- #-prefixed issue number(s), if any -->
| License       | MIT
| Doc PR        | symfony/symfony-docs#... <!--highly recommended for new features-->

In my project I get assert constraints from one of my entity and I use this value in my front end via Twig.
I faced a problem with the property $maxSize of the File Constraint.

As this property is protected I cannot access it via Twig because the magic method __isset is missing, as I read in twig documentation.
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Commits
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9efb765 [Validator] added magic method __isset()  to File Constraint class

Author: fabpot

src/Symfony/Component/Validator/Constraints/File.php

@@ -88,6 +88,15 @@ public function __get($option)
         return parent::__get($option);
     }
 
+    public function __isset($option)
+    {
+        if ('maxSize' === $option) {
+            return true;
+        }
+
+        return parent::__isset($option);
+    }
+
     private function normalizeBinaryFormat($maxSize)
     {
         $sizeInt = (int) $maxSize;