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

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

 
 class _HTMLOptionsCollectionJs extends _HTMLCollectionJs implements HTMLOptionsCollection native "*HTMLOptionsCollection" {
 
-  int get length() native "return this.length;";
+  int length;
 
-  void set length(int value) native "this.length = value;";
-
-  int get selectedIndex() native "return this.selectedIndex;";
-
-  void set selectedIndex(int value) native "this.selectedIndex = value;";
+  int selectedIndex;
 
   void remove(int index) native;
 }