Where is ScriptObjectBuilder?

Hello, 

I am attempting to update AJAX Control Toolkit from v3 to v15. I have some legacy code that relies on ScriptObjectBuilder. More specifically, the code calls ScriptObjectBuilder.GetScriptReferences. 

IEnumerable<ScriptReference> IScriptControl.GetScriptReferences()
{
  foreach (ScriptReference modalReference in ScriptObjectBuilder.GetScriptReferences(typeof(ModalPopupExtender)))
                yield return modalReference;

  // Yields other script references
}

How should I update this code to work with v15? 

Thank you