pkg/unittest: simplify guardAsync

There are features needed for unittest impl that don't make a lot of sense for users.

BUG=https://code.google.com/p/dart/issues/detail?id=8878
Committed: https://code.google.com/p/dart/source/detail?r=19347

[kevmoo-old, 2013-03-01 21:48:05]

PTAL

[gram, 2013-03-02 00:22:31]

lgtm. Did you verify that no other test in the tree uses the finally parameter
to guardAsync?

[kevmoo-old, 2013-03-02 00:27:31]

The only user was impl in unittest.

I walked through the grep of the other uses and verified

./tools/bots/pub.py -m pub-mac

was green

[gram, 2013-03-02 01:08:37]

Committed patchset #1 manually as r19347 (presubmit successful).