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

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

 
 class _HTMLCanvasElementJs extends _HTMLElementJs implements HTMLCanvasElement native "*HTMLCanvasElement" {
 
-  int get height() native "return this.height;";
+  int height;
 
-  void set height(int value) native "this.height = value;";
-
-  int get width() native "return this.width;";
-
-  void set width(int value) native "this.width = value;";
+  int width;
 
   Object getContext(String contextId) native;
 
   String toDataURL(String type) native;
 }