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

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

Index: client/dom/generated/src/frog/MessageEvent.dart
diff --git a/client/dom/generated/src/frog/MessageEvent.dart b/client/dom/generated/src/frog/MessageEvent.dart
index d67720cfd8b019e189982f32c43ffaa0daae7ccd..3b6e9bfac07cbf1e71383b98066a91a49ffab61e 100644
--- a/client/dom/generated/src/frog/MessageEvent.dart
+++ b/client/dom/generated/src/frog/MessageEvent.dart
@@ -1,5 +1,5 @@
 
-class MessageEventJs extends EventJs implements MessageEvent native "*MessageEvent" {
+class _MessageEventJs extends _EventJs implements MessageEvent native "*MessageEvent" {
 
   Object get data() native "return this.data;";
 
@@ -9,9 +9,9 @@ class MessageEventJs extends EventJs implements MessageEvent native "*MessageEve
 
   List get ports() native "return this.ports;";
 
-  DOMWindowJs get source() native "return this.source;";
+  _DOMWindowJs get source() native "return this.source;";
 
-  void initMessageEvent(String typeArg, bool canBubbleArg, bool cancelableArg, Object dataArg, String originArg, String lastEventIdArg, DOMWindowJs sourceArg, List messagePorts) native;
+  void initMessageEvent(String typeArg, bool canBubbleArg, bool cancelableArg, Object dataArg, String originArg, String lastEventIdArg, _DOMWindowJs sourceArg, List messagePorts) native;
 
-  void webkitInitMessageEvent(String typeArg, bool canBubbleArg, bool cancelableArg, Object dataArg, String originArg, String lastEventIdArg, DOMWindowJs sourceArg, List transferables) native;
+  void webkitInitMessageEvent(String typeArg, bool canBubbleArg, bool cancelableArg, Object dataArg, String originArg, String lastEventIdArg, _DOMWindowJs sourceArg, List transferables) native;
 }