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

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

Index: client/dom/generated/src/frog/HTMLFormElement.dart
diff --git a/client/dom/generated/src/frog/HTMLFormElement.dart b/client/dom/generated/src/frog/HTMLFormElement.dart
index e6da253e5a22d7dab821e77f6278205c1e09e7f1..81380fd8dc56413320ab434b8b89427810613e1d 100644
--- a/client/dom/generated/src/frog/HTMLFormElement.dart
+++ b/client/dom/generated/src/frog/HTMLFormElement.dart
@@ -1,5 +1,5 @@
 
-class HTMLFormElementJs extends HTMLElementJs implements HTMLFormElement native "*HTMLFormElement" {
+class _HTMLFormElementJs extends _HTMLElementJs implements HTMLFormElement native "*HTMLFormElement" {
 
   String get acceptCharset() native "return this.acceptCharset;";
 
@@ -13,7 +13,7 @@ class HTMLFormElementJs extends HTMLElementJs implements HTMLFormElement native
 
   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;";