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

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

Index: client/dom/generated/src/frog/HTMLTrackElement.dart
diff --git a/client/dom/generated/src/frog/HTMLTrackElement.dart b/client/dom/generated/src/frog/HTMLTrackElement.dart
index e093d40d9cbb94fb1d9cda7c4e0ac3bf39c3ed53..80671b78c5923a3fe6b3ab3a3b234c5cfaa8f15f 100644
--- a/client/dom/generated/src/frog/HTMLTrackElement.dart
+++ b/client/dom/generated/src/frog/HTMLTrackElement.dart
@@ -1,5 +1,5 @@
 
-class HTMLTrackElementJs extends HTMLElementJs implements HTMLTrackElement native "*HTMLTrackElement" {
+class _HTMLTrackElementJs extends _HTMLElementJs implements HTMLTrackElement native "*HTMLTrackElement" {
 
   static final int ERROR = 3;
 
@@ -31,5 +31,5 @@ class HTMLTrackElementJs extends HTMLElementJs implements HTMLTrackElement nativ
 
   void set srclang(String value) native "this.srclang = value;";
 
-  TextTrackJs get track() native "return this.track;";
+  _TextTrackJs get track() native "return this.track;";
 }