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

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

 
 class _HTMLCollectionJs extends _DOMTypeJs implements HTMLCollection native "*HTMLCollection" {
 
-  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 58 matching lines @@
   }
   List<Node> getRange(int start, int length) =>
       _Lists.getRange(this, start, length, <Node>[]);
 
   // -- end List<Node> mixins.
 
   _NodeJs item(int index) native;
 
   _NodeJs namedItem(String name) native;
 }