Make common elements publicly accessible.  

Most notably, root objects like ui.Widget, ui.List, and data.DataSource objects, and sub functions like parseOption and parseOptions, are all internally referenced.  You cannot extend any of them to provide global functionality to the elements that inherit from them.

kendo.ui.List and kendo.data.DataSource were particularly frustrating to debug, since the externally available versions are only copies of the real internal objects.  I tried to add global functionality that never worked until I realized that the real inherited objects were closure objects.