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

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

 
 class _HTMLLabelElementJs extends _HTMLElementJs implements HTMLLabelElement native "*HTMLLabelElement" {
 
-  _HTMLElementJs get control() native "return this.control;";
+  final _HTMLElementJs control;
 
-  _HTMLFormElementJs get form() native "return this.form;";
+  final _HTMLFormElementJs form;
 
-  String get htmlFor() native "return this.htmlFor;";
-
-  void set htmlFor(String value) native "this.htmlFor = value;";
+  String htmlFor;
 }