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

sky/engine/tonic/dart_state.cc

Index: sky/engine/tonic/dart_state.cc
diff --git a/sky/engine/tonic/dart_state.cc b/sky/engine/tonic/dart_state.cc
index 70faa05cd66d23e4fbbb0cf8a9c380c5c75ed6ac..c5bcdff71ebb2a3fd187fa3499c37d24724c37d3 100644
--- a/sky/engine/tonic/dart_state.cc
+++ b/sky/engine/tonic/dart_state.cc
@@ -6,6 +6,7 @@
 #include "sky/engine/tonic/dart_state.h"
 
 #include "sky/engine/tonic/dart_class_library.h"
+#include "sky/engine/tonic/dart_converter.h"
 #include "sky/engine/tonic/dart_exception_factory.h"
 #include "sky/engine/tonic/dart_string_cache.h"
 #include "sky/engine/tonic/dart_timer_heap.h"
@@ -31,6 +32,14 @@ DartState::DartState()
 DartState::~DartState() {
 }
 
+void DartState::DidSetIsolateInternal() {
+  {
+    Scope dart_scope(this);
+    index_handle_.Set(this, ToDart("index"));
+  }
+  DidSetIsolate();
+}
+
 DartState* DartState::From(Dart_Isolate isolate) {
   return static_cast<DartState*>(Dart_IsolateData(isolate));
 }