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

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

 
-class HTMLOutputElementJs extends HTMLElementJs implements HTMLOutputElement native "*HTMLOutputElement" {
+class _HTMLOutputElementJs extends _HTMLElementJs implements HTMLOutputElement native "*HTMLOutputElement" {
 
   String get defaultValue() native "return this.defaultValue;";
 
   void set defaultValue(String value) native "this.defaultValue = value;";
 
-  HTMLFormElementJs get form() native "return this.form;";
+  _HTMLFormElementJs get form() native "return this.form;";
 
-  DOMSettableTokenListJs get htmlFor() native "return this.htmlFor;";
+  _DOMSettableTokenListJs get htmlFor() native "return this.htmlFor;";
 
-  void set htmlFor(DOMSettableTokenListJs value) native "this.htmlFor = value;";
+  void set htmlFor(_DOMSettableTokenListJs value) native "this.htmlFor = 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 setCustomValidity(String error) native;
 }