[co19] Enable some LibTest/io tests
[co19] Enable some LibTest/io tests

Change-Id: I6c05c1554634e0bcf1dd0371e4425f5a1b55556a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/159844
Reviewed-by: Alexander Thomas <29642742b6693024c89de8232f2e2542cf7eedf7@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk