Sky: Convert TransferMode to an enum.

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 11e7bebf1aba37e9af3a3ffb0577668ff94b2220..5f22b6feabbabfac40fbf042656254f6cb27cf91 100644
--- a/sky/engine/core/script/dom_dart_state.h
+++ b/sky/engine/core/script/dom_dart_state.h
@@ -34,6 +34,7 @@ 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(); }
 
@@ -43,6 +44,7 @@ class DOMDartState : public DartState {
 
   DartPersistentValue x_handle_;
   DartPersistentValue y_handle_;
+  DartPersistentValue index_handle_;
   DartPersistentValue value_handle_;
   DartPersistentValue color_class_;
 };