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

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

 
 class _DOMTokenListJs extends _DOMTypeJs implements DOMTokenList native "*DOMTokenList" {
 
-  int get length() native "return this.length;";
+  final int length;
 
   void add(String token) native;
 
   bool contains(String token) native;
 
   String item(int index) native;
 
   void remove(String token) native;
 
   String toString() native;
 
   bool toggle(String token) native;
 }