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

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

 
 class _SVGStringListJs extends _DOMTypeJs implements SVGStringList native "*SVGStringList" {
 
-  int get numberOfItems() native "return this.numberOfItems;";
+  final int numberOfItems;
 
   String appendItem(String item) native;
 
   void clear() native;
 
   String getItem(int index) native;
 
   String initialize(String item) native;
 
   String insertItemBefore(String item, int index) native;
 
   String removeItem(int index) native;
 
   String replaceItem(String item, int index) native;
 }