Try to fix some flaky tests (#150)

The suite of common tests repeatedly opens `IOSink`s and
`StreamController`s but closed them only the first time.  I don't
definitively know that this was the cause for many of the test flakes
related to local filesystems, but with this change, I haven't hit
file-not-found flakes locally after several hundred iterations of
running `chroot_test.dart`.
1 file changed