Morph the APIs for Node, ParentNode, and Element closer to the specs

sky/engine/tonic/dart_wrappable.h

Index: sky/engine/tonic/dart_wrappable.h
diff --git a/sky/engine/tonic/dart_wrappable.h b/sky/engine/tonic/dart_wrappable.h
index dba0f182b0275aeac62ca01d9b743337791da0a3..f070d7f50576ac9c4690b68f32a1dfc1193bca13 100644
--- a/sky/engine/tonic/dart_wrappable.h
+++ b/sky/engine/tonic/dart_wrappable.h
@@ -126,6 +126,10 @@ struct DartConverter<RefPtr<T>> {
   static Dart_Handle ToDart(RefPtr<T> val) {
     return DartConverter<T*>::ToDart(val.get());
   }
+
+  static RefPtr<T> FromDart(Dart_Handle handle) {
+    return DartConverter<T*>::FromDart(handle);
+  }
 };
 
 template<typename T>