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..42ef6d5004d7f731d6e2c9b85f95f42d78a9576d 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 +32,13 @@ DartState::DartState()
 DartState::~DartState() {
 }
 
+void DartState::DidSetIsolateInternal() {
+  Scope dart_scope(this);
+  index_handle_.Set(this, ToDart("index"));
+
+  DidSetIsolate();

[abarth-chromium, 2015/06/09 19:51:37]

Do we want dart_scope to be on the stack while we're calling DidSetIsolate? 
Just asking.  I'm not sure one way or another.  It just seems like a change from
what we had previously.

[Matt Perry, 2015/06/09 19:52:47]

Yeah I could go either way.

[Matt Perry, 2015/06/09 19:55:39]

Went back to the old way since it's less mysterious.

+}
+
 DartState* DartState::From(Dart_Isolate isolate) {
   return static_cast<DartState*>(Dart_IsolateData(isolate));
 }