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

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

 
-class HTMLButtonElementJs extends HTMLElementJs implements HTMLButtonElement native "*HTMLButtonElement" {
+class _HTMLButtonElementJs extends _HTMLElementJs implements HTMLButtonElement native "*HTMLButtonElement" {
 
   bool get autofocus() native "return this.autofocus;";
 
   void set autofocus(bool value) native "this.autofocus = 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;";
 
   String get formAction() native "return this.formAction;";
 
   void set formAction(String value) native "this.formAction = value;";
 
   String get formEnctype() native "return this.formEnctype;";
 
   void set formEnctype(String value) native "this.formEnctype = value;";
 
   String get formMethod() native "return this.formMethod;";
 
   void set formMethod(String value) native "this.formMethod = value;";
 
   bool get formNoValidate() native "return this.formNoValidate;";
 
   void set formNoValidate(bool value) native "this.formNoValidate = value;";
 
   String get formTarget() native "return this.formTarget;";
 
   void set formTarget(String value) native "this.formTarget = value;";
 
-  NodeListJs get labels() native "return this.labels;";
+  _NodeListJs get labels() native "return this.labels;";
 
   String get name() native "return this.name;";
 
   void set name(String value) native "this.name = value;";
 
   String get type() native "return this.type;";
 
   String get validationMessage() native "return this.validationMessage;";
 
-  ValidityStateJs get validity() native "return this.validity;";
+  _ValidityStateJs get validity() native "return this.validity;";
 
   String get value() native "return this.value;";
 
   void set value(String value) native "this.value = value;";
 
   bool get willValidate() native "return this.willValidate;";
 
   bool checkValidity() native;
 
   void click() native;
 
   void setCustomValidity(String error) native;
 }