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

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

Index: client/dom/generated/src/frog/CSSRule.dart
diff --git a/client/dom/generated/src/frog/CSSRule.dart b/client/dom/generated/src/frog/CSSRule.dart
index aebe849b7f31b92e62cdc61921f935b3e9bb2e8b..e1c22a992a21cf035283998de2eb57a2980de0a6 100644
--- a/client/dom/generated/src/frog/CSSRule.dart
+++ b/client/dom/generated/src/frog/CSSRule.dart
@@ -1,5 +1,5 @@
 
-class CSSRuleJs extends DOMTypeJs implements CSSRule native "*CSSRule" {
+class _CSSRuleJs extends _DOMTypeJs implements CSSRule native "*CSSRule" {
 
   static final int CHARSET_RULE = 2;
 
@@ -25,9 +25,9 @@ class CSSRuleJs extends DOMTypeJs implements CSSRule native "*CSSRule" {
 
   void set cssText(String value) native "this.cssText = value;";
 
-  CSSRuleJs get parentRule() native "return this.parentRule;";
+  _CSSRuleJs get parentRule() native "return this.parentRule;";
 
-  CSSStyleSheetJs get parentStyleSheet() native "return this.parentStyleSheet;";
+  _CSSStyleSheetJs get parentStyleSheet() native "return this.parentStyleSheet;";
 
   int get type() native "return this.type;";
 }