Upgrade to Ajax Toolkit 2013: AjaxControlToolkit' is undefined

I upgraded from an earlier version Ajax toolkit AjaxControlToolkit.dll (3/33/2012 date) to the new Version  dll date 6/15/2012. Most everything in the site seems to run OK except I gte  this error:

Microsoft JScript runtime error: 'AjaxControlToolkit' is undefined on this line

Hero.Common.Controls.ModalPopupBehavior.registerClass('Hero.Common.Controls.ModalPopupBehavior', AjaxControlToolkit.ModalPopupBehavior);

If I put the old version back the error goes away. What did I miss?

Thanks