bug symfony#25179 [FrameworkBundle][Serializer] Remove YamlEncoder definition if Yaml component isn't installed (ogizanagi)

This PR was merged into the 3.3 branch.

Discussion
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[FrameworkBundle][Serializer] Remove YamlEncoder definition if Yaml component isn't installed

| Q             | A
| ------------- | ---
| Branch?       | 3.3 <!-- see below -->
| Bug fix?      | yes
| New feature?  | no <!-- 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#24634 (comment) <!-- #-prefixed issue number(s), if any -->
| License       | MIT
| Doc PR        | N/A

Do we need a `"symfony/yaml": "<3.2"` conflict here (the version used in `require-dev`)? Otherwise I'll re-introduce the `!defined('Symfony\Component\Yaml\Yaml::DUMP_OBJECT')` check instead.

Commits
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a44f8a5 [FrameworkBundle][Serializer] Remove YamlEncoder definition if Yaml component isn't installed

Author: fabpot

src/Symfony/Bundle/FrameworkBundle/DependencyInjection/FrameworkExtension.php

@@ -56,6 +56,7 @@
 use Symfony\Component\Validator\ObjectInitializerInterface;
 use Symfony\Component\WebLink\HttpHeaderSerializer;
 use Symfony\Component\Workflow;
+use Symfony\Component\Yaml\Yaml;
 
 /**
  * FrameworkExtension.
@@ -1172,6 +1173,10 @@ private function registerSerializerConfiguration(array $config, ContainerBuilder
             $container->removeDefinition('serializer.normalizer.object');
         }
 
+        if (!class_exists(Yaml::class)) {
+            $container->removeDefinition('serializer.encoder.yaml');
+        }
+
         $serializerLoaders = array();
         if (isset($config['enable_annotations']) && $config['enable_annotations']) {
             if (!$this->annotationsConfigEnabled) {