Sky: Add a DartConverterEnum and use that for all our enum needs.

sky/engine/core/script/dom_dart_state.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef SKY_ENGINE_CORE_SCRIPT_DOM_DART_STATE_H_
 #define SKY_ENGINE_CORE_SCRIPT_DOM_DART_STATE_H_
 
 #include "dart/runtime/include/dart_api.h"
 #include "sky/engine/tonic/dart_state.h"
 #include "sky/engine/core/dom/Document.h"
@@ skipping 18 matching lines @@
   static LocalDOMWindow* CurrentWindow();
 
   const KURL& url() const { return url_; }
 
   Document* document() const { return document_.get(); }
   DartLoader& loader() const { return *loader_; }
 
   // Cached handles to strings used in Dart/C++ conversions.
   Dart_Handle x_handle() { return x_handle_.value(); }
   Dart_Handle y_handle() { return y_handle_.value(); }
-  Dart_Handle index_handle() { return index_handle_.value(); }
   Dart_Handle value_handle() { return value_handle_.value(); }
   Dart_Handle color_class() { return color_class_.value(); }
 
  private:
   RefPtr<Document> document_;
   KURL url_;
   OwnPtr<DartLoader> loader_;
 
   DartPersistentValue x_handle_;
   DartPersistentValue y_handle_;
-  DartPersistentValue index_handle_;
   DartPersistentValue value_handle_;
   DartPersistentValue color_class_;
 };
 
 }  // namespace blink
 
 #endif // SKY_ENGINE_CORE_SCRIPT_DOM_DART_STATE_H_