I noticed recently that progress messages were no longer displayed (though
the ASCII spinner was still animating). I bisected the problem to the
progress.ts conversion in #10650, and I attempted to reproduce the
conversion, step by step. The problem ended up being the renaming of
._title to .title, which was a mistake because external code accesses
progress._title. Once I updated those invasive access points, progress
messages began displaying once again.
