Commented Unassigned: ScriptObjectBuilder.GetCssReferences [27810]

I am using AjaxControlToolkit3.5, in this version, i can use ScriptObjectBuilder.GetCssReferences to get cssReference of an extender.
Now, i update to use version 15.1, and i can't find ScriptObjectBuilder.
What can i do to GetCssReferences. Thanks
Comments: Do you have any suggestion?