ES6 symbols: Allow symbols as property names

src/json-stringifier.h

Index: src/json-stringifier.h
diff --git a/src/json-stringifier.h b/src/json-stringifier.h
index b683a46db2667eba0d6a077c06b629a49310cca6..669de069371d4b0d2a92ea32373f14c7c103e811 100644
--- a/src/json-stringifier.h
+++ b/src/json-stringifier.h
@@ -540,7 +540,10 @@ BasicJsonStringifier::Result BasicJsonStringifier::SerializeJSObject(
       object->elements()->length() == 0) {
     Handle<Map> map(object->map());
     for (int i = 0; i < map->NumberOfOwnDescriptors(); i++) {
-      Handle<String> key(map->instance_descriptors()->GetKey(i), isolate_);
+      Handle<Name> name(map->instance_descriptors()->GetKey(i), isolate_);
+      // TODO(rossberg): Should this throw?
+      if (!name->IsString()) continue;
+      Handle<String> key = Handle<String>::cast(name);
       PropertyDetails details = map->instance_descriptors()->GetDetails(i);
       if (details.IsDontEnum() || details.IsDeleted()) continue;
       Handle<Object> property;