feature symfony#25295 [Translation] Parse PHP constants in YAML translation files (ostrolucky)

This PR was merged into the 4.1-dev branch.

Discussion
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[Translation] Parse PHP constants in YAML translation files

| 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
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d2c8532 [Translator] Parse PHP constants in YAML translation files

Author: fabpot

src/Symfony/Component/Translation/Loader/YamlFileLoader.php

@@ -15,6 +15,7 @@
 use Symfony\Component\Translation\Exception\LogicException;
 use Symfony\Component\Yaml\Parser as YamlParser;
 use Symfony\Component\Yaml\Exception\ParseException;
+use Symfony\Component\Yaml\Yaml;
 
 /**
  * YamlFileLoader loads translations from Yaml files.
@@ -39,7 +40,7 @@ protected function loadResource($resource)
         }
 
         try {
-            $messages = $this->yamlParser->parseFile($resource);
+            $messages = $this->yamlParser->parseFile($resource, Yaml::PARSE_CONSTANT);
         } catch (ParseException $e) {
             throw new InvalidResourceException(sprintf('Error parsing YAML, invalid file "%s"', $resource), 0, $e);
         }