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

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

 
-class MediaListJs extends DOMTypeJs implements MediaList native "*MediaList" {
+class _MediaListJs extends _DOMTypeJs implements MediaList native "*MediaList" {
 
   int get length() native "return this.length;";
 
   String get mediaText() native "return this.mediaText;";
 
   void set mediaText(String value) native "this.mediaText = value;";
 
   String operator[](int index) native "return this[index];";
 
   void operator[]=(int index, String value) {
@@ skipping 66 matching lines @@
       _Lists.getRange(this, start, length, <String>[]);
 
   // -- end List<String> mixins.
 
   void appendMedium(String newMedium) native;
 
   void deleteMedium(String oldMedium) native;
 
   String item(int index) native;
 }