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

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

 
 class _NamedNodeMapJs extends _DOMTypeJs implements NamedNodeMap native "*NamedNodeMap" {
 
-  int get length() native "return this.length;";
+  final int length;
 
   _NodeJs operator[](int index) native "return this[index];";
 
   void operator[]=(int index, _NodeJs value) {
     throw new UnsupportedOperationException("Cannot assign element of immutable List.");
   }
   // -- start List<Node> mixins.
   // Node is the element type.
 
   // From Iterable<Node>:
@@ skipping 68 matching lines @@
   _NodeJs item(int index) native;
 
   _NodeJs removeNamedItem(String name) native;
 
   _NodeJs removeNamedItemNS(String namespaceURI, String localName) native;
 
   _NodeJs setNamedItem(_NodeJs node) native;
 
   _NodeJs setNamedItemNS(_NodeJs node) native;
 }