bug #17672 [DependencyInjection][Routing] add files used in FileResource objects (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
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[DependencyInjection][Routing] add files used in FileResource objects

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | https://github.com/symfony/symfony/pull/17598#discussion_r51756307
| License       | MIT
| Doc PR        |

Starting with Symfony 3.1, the constructor of the `FileResource` class
will throw an exception when the passed file does not exist.

Commits
-------

4f865c7 add files used in FileResource objects

src/Symfony/Component/DependencyInjection/ContainerBuilder.php

@@ -243,7 +243,9 @@ class ContainerBuilder extends Container implements TaggedContainerInterface
         }
 
         do {
-            $this->addResource(new FileResource($class->getFileName()));
+            if (is_file($class->getFileName())) {
+                $this->addResource(new FileResource($class->getFileName()));
+            }
         } while ($class = $class->getParentClass());
 
         return $this;