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

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

 
 class _HTMLPreElementJs extends _HTMLElementJs implements HTMLPreElement native "*HTMLPreElement" {
 
-  int get width() native "return this.width;";
+  int width;
 
-  void set width(int value) native "this.width = value;";
-
-  bool get wrap() native "return this.wrap;";
-
-  void set wrap(bool value) native "this.wrap = value;";
+  bool wrap;
 }