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

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

 
 class _StyleSheetListJs extends _DOMTypeJs implements StyleSheetList native "*StyleSheetList" {
 
-  int get length() native "return this.length;";
+  final int length;
 
   _StyleSheetJs operator[](int index) native "return this[index];";
 
   void operator[]=(int index, _StyleSheetJs value) {
     throw new UnsupportedOperationException("Cannot assign element of immutable List.");
   }
   // -- start List<StyleSheet> mixins.
   // StyleSheet is the element type.
 
   // From Iterable<StyleSheet>:
@@ skipping 56 matching lines @@
   void insertRange(int start, int length, [StyleSheet initialValue]) {
     throw new UnsupportedOperationException("Cannot insertRange on immutable List.");
   }
   List<StyleSheet> getRange(int start, int length) =>
       _Lists.getRange(this, start, length, <StyleSheet>[]);
 
   // -- end List<StyleSheet> mixins.
 
   _StyleSheetJs item(int index) native;
 }