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

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

 
 class _HTMLLIElementJs extends _HTMLElementJs implements HTMLLIElement native "*HTMLLIElement" {
 
-  String get type() native "return this.type;";
+  String type;
 
-  void set type(String value) native "this.type = value;";
-
-  int get value() native "return this.value;";
-
-  void set value(int value) native "this.value = value;";
+  int value;
 }