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

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

 
-class HTMLCanvasElementJs extends HTMLElementJs implements HTMLCanvasElement native "*HTMLCanvasElement" {
+class _HTMLCanvasElementJs extends _HTMLElementJs implements HTMLCanvasElement native "*HTMLCanvasElement" {
 
   int get height() native "return this.height;";
 
   void set height(int value) native "this.height = value;";
 
   int get width() native "return this.width;";
 
   void set width(int value) native "this.width = value;";
 
   Object getContext(String contextId) native;
 
   String toDataURL(String type) native;
 }