Error "editor.ui.addButton is not a function", ONLY with development version

Hi,

I recently faced the problem of an unexpected error coming from CKEditor development version, while it didn't fire with standard version:

TypeError: editor.ui.addButton is not a function

(more details about this: https://www.drupal.org/node/2138969)

Diving in sources, I discovered that core/ui.js defines only some basic methods, while additonal ones come from other parts.

Notably regarding my problem, I notice that core/ui.js defines ui.add(name, type, definition), while plugins/button/plugin.js is responsible to define ui.addButton(name, definition) which in turn calls ui.add(name,CKEDITOR.UI_BUTTON, definition)

So by feel, I suspect some delay before the latter method is "known"... but I can't figure out really what happens.

Is there something obvious I'm missing?

Thanks in advance for any comment.