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

sky/engine/core/script/dom_dart_state.h

Index: sky/engine/core/script/dom_dart_state.h
diff --git a/sky/engine/core/script/dom_dart_state.h b/sky/engine/core/script/dom_dart_state.h
index f34f5467cd392cb6ccc2da675573ccd2ea940645..7381ef6671ee674e7ab67a768e3a785f6ceb9630 100644
--- a/sky/engine/core/script/dom_dart_state.h
+++ b/sky/engine/core/script/dom_dart_state.h
@@ -36,7 +36,6 @@ class DOMDartState : public DartState {
   // 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(); }
 
@@ -47,7 +46,6 @@ class DOMDartState : public DartState {
 
   DartPersistentValue x_handle_;
   DartPersistentValue y_handle_;
-  DartPersistentValue index_handle_;
   DartPersistentValue value_handle_;
   DartPersistentValue color_class_;
 };