Change all the places that still used old-style asynchronous async in
Fasta. These were mostly in testing.
Change-Id: I102209aa54c7c6e658b9d6ec98167af62bf12f3e
Reviewed-on: https://dart-review.googlesource.com/c/87074
Reviewed-by: Peter von der Ahé <ahe@google.com>
Commit-Queue: Kevin Millikin <kmillikin@google.com>