flip back the order of key check to get tests passing

Author: kevmoo

json_annotation/lib/src/allowed_keys_helpers.dart

@@ -20,20 +20,20 @@ void $checkAllowedKeys(Map map, Iterable<String> allowedKeys) {
 /// Should not be used directly.
 void $checkKeys(Map map,
     {List<String> allowedKeys, List<String> requiredKeys}) {
-  if (requiredKeys != null) {
-    var missingKeys = requiredKeys.where((k) => !map.keys.contains(k)).toList();
-    if (missingKeys.isNotEmpty) {
-      throw new MissingRequiredKeysException(missingKeys, map);
-    }
-  }
-
   if (map != null && allowedKeys != null) {
     var invalidKeys =
         map.keys.cast<String>().where((k) => !allowedKeys.contains(k)).toList();
     if (invalidKeys.isNotEmpty) {
       throw new UnrecognizedKeysException(invalidKeys, map, allowedKeys);
     }
   }
+
+  if (requiredKeys != null) {
+    var missingKeys = requiredKeys.where((k) => !map.keys.contains(k)).toList();
+    if (missingKeys.isNotEmpty) {
+      throw new MissingRequiredKeysException(missingKeys, map);
+    }
+  }
 }
 
 /// Exception thrown if there are unrecognized keys in a JSON map that was