Wrong/Incompatible field used for spellcheck index creation

I just realized that when fixing the spellchecker (#1859232: Make the naming for the extra included schema types and fields more general as we expect, according to a quick git bisect) you used content as the source field for the spellcheck index. This field isn't used by the Search API, we used spell as the source for the spellchecking.
I don't know whether the change was deliberate, or an oversight, but in any case it breaks spellchecking in the Search API. So can we please switch back to using spell again, which is available for both modules?