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

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

 
-class MutationEventJs extends EventJs implements MutationEvent native "*MutationEvent" {
+class _MutationEventJs extends _EventJs implements MutationEvent native "*MutationEvent" {
 
   static final int ADDITION = 2;
 
   static final int MODIFICATION = 1;
 
   static final int REMOVAL = 3;
 
   int get attrChange() native "return this.attrChange;";
 
   String get attrName() native "return this.attrName;";
 
   String get newValue() native "return this.newValue;";
 
   String get prevValue() native "return this.prevValue;";
 
-  NodeJs get relatedNode() native "return this.relatedNode;";
+  _NodeJs get relatedNode() native "return this.relatedNode;";
 
-  void initMutationEvent(String type, bool canBubble, bool cancelable, NodeJs relatedNode, String prevValue, String newValue, String attrName, int attrChange) native;
+  void initMutationEvent(String type, bool canBubble, bool cancelable, _NodeJs relatedNode, String prevValue, String newValue, String attrName, int attrChange) native;
 }