Merge branch '3.4' into 4.0

* 3.4:
  [HttpKernel] Fix race condition when clearing old containers
  [DI] Fix infinite loop in InlineServiceDefinitionsPass
  [HttpKernel] Keep legacy container files for concurrent requests
  Do not cache cache attributes if `attributes` is in the context
  Test that it do not remove the new flashes when displaying the existing ones
  [HttpFoundation] AutExpireFlashBag should not clear new flashes
  [FrameworkBundle][Serializer] Remove YamlEncoder definition if Yaml component isn't installed
  [DI] Fix tracking of env vars in exceptions
  [Form] Don't rely on  if http-foundation isn't in FileType
  Fix merge
  substitute aliases in inline mappings
  added ability for substitute aliases when mapping in YAML is on single line
  [Console] Fix global console flag when used in chain

Author: xabbuh

DependencyInjection/FrameworkExtension.php

@@ -74,6 +74,7 @@
 use Symfony\Component\WebLink\HttpHeaderSerializer;
 use Symfony\Component\Workflow;
 use Symfony\Component\Yaml\Command\LintCommand as BaseYamlLintCommand;
+use Symfony\Component\Yaml\Yaml;
 
 /**
  * FrameworkExtension.
@@ -1159,6 +1160,10 @@ 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) {