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

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

 
 class _ImageDataJs extends _DOMTypeJs implements ImageData native "*ImageData" {
 
-  _CanvasPixelArrayJs get data() native "return this.data;";
+  final _CanvasPixelArrayJs data;
 
-  int get height() native "return this.height;";
+  final int height;
 
-  int get width() native "return this.width;";
+  final int width;
 }