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

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

 
-class HTMLAreaElementJs extends HTMLElementJs implements HTMLAreaElement native "*HTMLAreaElement" {
+class _HTMLAreaElementJs extends _HTMLElementJs implements HTMLAreaElement native "*HTMLAreaElement" {
 
   String get alt() native "return this.alt;";
 
   void set alt(String value) native "this.alt = value;";
 
   String get coords() native "return this.coords;";
 
   void set coords(String value) native "this.coords = value;";
 
   String get hash() native "return this.hash;";
@@ skipping 23 matching lines @@
   String get search() native "return this.search;";
 
   String get shape() native "return this.shape;";
 
   void set shape(String value) native "this.shape = value;";
 
   String get target() native "return this.target;";
 
   void set target(String value) native "this.target = value;";
 }