feature symfony#25294 [Serializer] Parse PHP constants in YAML mappings (ostrolucky)

This PR was merged into the 4.1-dev branch.

Discussion
----------

[Serializer] Parse PHP constants in YAML mappings

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

Commits
-------

b82f301 [Serializer] Parse PHP constants in YAML mappings

Author: fabpot

src/Symfony/Component/Serializer/Mapping/Loader/YamlFileLoader.php

@@ -15,6 +15,7 @@
 use Symfony\Component\Serializer\Mapping\AttributeMetadata;
 use Symfony\Component\Serializer\Mapping\ClassMetadataInterface;
 use Symfony\Component\Yaml\Parser;
+use Symfony\Component\Yaml\Yaml;
 
 /**
  * YAML File Loader.
@@ -113,7 +114,7 @@ private function getClassesFromYaml()
             $this->yamlParser = new Parser();
         }
 
-        $classes = $this->yamlParser->parseFile($this->file);
+        $classes = $this->yamlParser->parseFile($this->file, Yaml::PARSE_CONSTANT);
 
         if (empty($classes)) {
             return array();