It was causing stability issues because the first pass
could cause a "magic trailing comma" to appear, meaning
that the second pass might get a different result. It's
not critical.
Some things format differently (with extra parens)
Fixes#2651. Fixes#2754. Fixes#2518. Fixes#2321.
This adds a test that lists a number of cases of unstable formatting
that we have seen in the issue tracker. Checking it in will ensure
that we don't regress on these cases.