Upgrade Symbol implementation to match current ES6 behavior.

src/hydrogen.cc

Index: src/hydrogen.cc
diff --git a/src/hydrogen.cc b/src/hydrogen.cc
index 478d938cf6fa27d74994d43674af59ca7f05ef41..8563c96d9d34374bf33bbf13bf438d21291b467f 100644
--- a/src/hydrogen.cc
+++ b/src/hydrogen.cc
@@ -1293,6 +1293,7 @@ HValue* HGraphBuilder::BuildCheckString(HValue* string) {
 
 HValue* HGraphBuilder::BuildWrapReceiver(HValue* object, HValue* function) {
   if (object->type().IsJSObject()) return object;
+  // FIXME: need to always wrap Symbol values (but no HType to test against..)
   if (function->IsConstant() &&
       HConstant::cast(function)->handle(isolate())->IsJSFunction()) {
     Handle<JSFunction> f = Handle<JSFunction>::cast(
@@ -5638,6 +5639,7 @@ bool HOptimizedGraphBuilder::PropertyAccessInfo::CanLoadAsMonomorphic(
 
 
 static bool NeedsWrappingFor(Handle<HeapType> type, Handle<JSFunction> target) {
+  if (type->Is(HeapType::Symbol())) return true;
   return type->Is(HeapType::NumberOrString()) &&
       target->shared()->is_classic_mode() &&
       !target->shared()->native();