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

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

Index: client/dom/generated/src/frog/Counter.dart
diff --git a/client/dom/generated/src/frog/Counter.dart b/client/dom/generated/src/frog/Counter.dart
index 70876200b595ed1f15afc5b7c2d635f2f46d642f..d0755959675031e6c8ce2c63195cca7a9751779e 100644
--- a/client/dom/generated/src/frog/Counter.dart
+++ b/client/dom/generated/src/frog/Counter.dart
@@ -1,9 +1,9 @@
 
 class _CounterJs extends _DOMTypeJs implements Counter native "*Counter" {
 
-  String get identifier() native "return this.identifier;";
+  final String identifier;
 
-  String get listStyle() native "return this.listStyle;";
+  final String listStyle;
 
-  String get separator() native "return this.separator;";
+  final String separator;
 }