With current master (f227fadb0f234cbd3c04110c1ce76abf740f283f of 28 March 2020):
The asserts namespace is used inside the function:
It is indeed loaded:
but the asserts reference inside goog.math.Long is evidently somehow not properly set up when the namespace is loaded via code near the top of that namespace that looks like:
The underlying issue is that we monkey patch isProvided_ to return false, but this prevents the goog.module branch case in the goog.require implementation and thus returns null.