Use fields in hidden native DOM classes instead of getters/setters.

client/dom/generated/src/frog/StorageEvent.dart

Index: client/dom/generated/src/frog/StorageEvent.dart
diff --git a/client/dom/generated/src/frog/StorageEvent.dart b/client/dom/generated/src/frog/StorageEvent.dart
index e9b04fad2057eb8b2fbdf9c122620833a86ea90f..a209ec8eaad6ba62746494a6d425b0d29736d648 100644
--- a/client/dom/generated/src/frog/StorageEvent.dart
+++ b/client/dom/generated/src/frog/StorageEvent.dart
@@ -1,15 +1,15 @@
 
 class _StorageEventJs extends _EventJs implements StorageEvent native "*StorageEvent" {
 
-  String get key() native "return this.key;";
+  final String key;
 
-  String get newValue() native "return this.newValue;";
+  final String newValue;
 
-  String get oldValue() native "return this.oldValue;";
+  final String oldValue;
 
-  _StorageJs get storageArea() native "return this.storageArea;";
+  final _StorageJs storageArea;
 
-  String get url() native "return this.url;";
+  final String url;
 
   void initStorageEvent(String typeArg, bool canBubbleArg, bool cancelableArg, String keyArg, String oldValueArg, String newValueArg, String urlArg, _StorageJs storageAreaArg) native;
 }