Make dart:dom implementation types private so they don't muddle the docs.

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

 
-class HTMLOptionElementJs extends HTMLElementJs implements HTMLOptionElement native "*HTMLOptionElement" {
+class _HTMLOptionElementJs extends _HTMLElementJs implements HTMLOptionElement native "*HTMLOptionElement" {
 
   bool get defaultSelected() native "return this.defaultSelected;";
 
   void set defaultSelected(bool value) native "this.defaultSelected = value;";
 
   bool get disabled() native "return this.disabled;";
 
   void set disabled(bool value) native "this.disabled = value;";
 
-  HTMLFormElementJs get form() native "return this.form;";
+  _HTMLFormElementJs get form() native "return this.form;";
 
   int get index() native "return this.index;";
 
   String get label() native "return this.label;";
 
   void set label(String value) native "this.label = value;";
 
   bool get selected() native "return this.selected;";
 
   void set selected(bool value) native "this.selected = value;";
 
   String get text() native "return this.text;";
 
   void set text(String value) native "this.text = value;";
 
   String get value() native "return this.value;";
 
   void set value(String value) native "this.value = value;";
 }