[es6] Implement spec compliant ToPrimitive in the runtime.

src/messages.cc

Index: src/messages.cc
diff --git a/src/messages.cc b/src/messages.cc
index a8afa3f5a75067c76c173d301c3d41b0a789a4bc..4dcac50bf2d63a56b3a1fb86f74a85272b381dd2 100644
--- a/src/messages.cc
+++ b/src/messages.cc
@@ -471,7 +471,7 @@ MaybeHandle<String> ErrorToStringHelper::GetStringifiedProperty(
                              String);
   if (obj->IsUndefined()) return default_value;
   if (!obj->IsString()) {
-    ASSIGN_RETURN_ON_EXCEPTION(isolate, obj, Execution::ToString(isolate, obj),
+    ASSIGN_RETURN_ON_EXCEPTION(isolate, obj, Object::ToString(isolate, obj),
                                String);
   }
   return Handle<String>::cast(obj);