The plugins are loaded right before theme (yes, I’ve been looking for excuse to use this):
However it is wrong to think about either as point of code execution. For most cases everything should be hooked and executed no earlier than
init hook. According to Codex widget registration with
register_widget() should be hooked to
Because of that order of load doesn’t matter for this case, you will have everything loaded by the time widget needs it in any case.
solved Between functions.php (theme), widgets, and plugins, which is loaded first?