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

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

 
 class _CanvasPixelArrayJs extends _DOMTypeJs implements CanvasPixelArray native "*CanvasPixelArray" {
 
-  int get length() native "return this.length;";
+  final int length;
 
   int operator[](int index) native "return this[index];";
 
   void operator[]=(int index, int value) native "this[index] = value";
   // -- start List<int> mixins.
   // int is the element type.
 
   // From Iterable<int>:
 
   Iterator<int> iterator() {
@@ skipping 52 matching lines @@
     throw new UnsupportedOperationException("Cannot removeRange on immutable List.");
   }
   void insertRange(int start, int length, [int initialValue]) {
     throw new UnsupportedOperationException("Cannot insertRange on immutable List.");
   }
   List<int> getRange(int start, int length) =>
       _Lists.getRange(this, start, length, <int>[]);
 
   // -- end List<int> mixins.
 }