Merge branch '5.1' into 5.2

* 5.1:
  Changed private static array-properties to const

Author: nicolas-grekas

Loader/YamlFileLoader.php

@@ -33,7 +33,7 @@ class YamlFileLoader extends FileLoader
     use LocalizedRouteTrait;
     use PrefixTrait;
 
-    private static $availableKeys = [
+    private const AVAILABLE_KEYS = [
         'resource', 'type', 'prefix', 'path', 'host', 'schemes', 'methods', 'defaults', 'requirements', 'options', 'condition', 'controller', 'name_prefix', 'trailing_slash_on_root', 'locale', 'format', 'utf8', 'exclude', 'stateless',
     ];
     private $yamlParser;
@@ -240,8 +240,8 @@ protected function validate($config, string $name, string $path)
         if (!\is_array($config)) {
             throw new \InvalidArgumentException(sprintf('The definition of "%s" in "%s" must be a YAML array.', $name, $path));
         }
-        if ($extraKeys = array_diff(array_keys($config), self::$availableKeys)) {
-            throw new \InvalidArgumentException(sprintf('The routing file "%s" contains unsupported keys for "%s": "%s". Expected one of: "%s".', $path, $name, implode('", "', $extraKeys), implode('", "', self::$availableKeys)));
+        if ($extraKeys = array_diff(array_keys($config), self::AVAILABLE_KEYS)) {
+            throw new \InvalidArgumentException(sprintf('The routing file "%s" contains unsupported keys for "%s": "%s". Expected one of: "%s".', $path, $name, implode('", "', $extraKeys), implode('", "', self::AVAILABLE_KEYS)));
         }
         if (isset($config['resource']) && isset($config['path'])) {
             throw new \InvalidArgumentException(sprintf('The routing file "%s" must not specify both the "resource" key and the "path" key for "%s". Choose between an import and a route definition.', $path, $name));