> This makes me wonder why we export ol.source.Source. Any idea?

Are you sure we export `ol.source.Source`?
```
$ find src -name \*.exports | xargs grep ol.source.Source
src/ol/source/source.exports:@exportProperty ol.source.Source.prototype.getExtent
$
```
i.e. currently only the `getExtent` property is exported, not the class.