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

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

 
 class _ProcessingInstructionJs extends _NodeJs implements ProcessingInstruction native "*ProcessingInstruction" {
 
-  String get data() native "return this.data;";
+  String data;
 
-  void set data(String value) native "this.data = value;";
+  final _StyleSheetJs sheet;
 
-  _StyleSheetJs get sheet() native "return this.sheet;";
-
-  String get target() native "return this.target;";
+  final String target;
 }