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

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

 
-class HTMLFormElementJs extends HTMLElementJs implements HTMLFormElement native "*HTMLFormElement" {
+class _HTMLFormElementJs extends _HTMLElementJs implements HTMLFormElement native "*HTMLFormElement" {
 
   String get acceptCharset() native "return this.acceptCharset;";
 
   void set acceptCharset(String value) native "this.acceptCharset = value;";
 
   String get action() native "return this.action;";
 
   void set action(String value) native "this.action = value;";
 
   String get autocomplete() native "return this.autocomplete;";
 
   void set autocomplete(String value) native "this.autocomplete = value;";
 
-  HTMLCollectionJs get elements() native "return this.elements;";
+  _HTMLCollectionJs get elements() native "return this.elements;";
 
   String get encoding() native "return this.encoding;";
 
   void set encoding(String value) native "this.encoding = value;";
 
   String get enctype() native "return this.enctype;";
 
   void set enctype(String value) native "this.enctype = value;";
 
   int get length() native "return this.length;";
@@ skipping 13 matching lines @@
   String get target() native "return this.target;";
 
   void set target(String value) native "this.target = value;";
 
   bool checkValidity() native;
 
   void reset() native;
 
   void submit() native;
 }