Fix issue where failing pub tests confused unittest.

Fix issue where failing pub tests confused unittest.

Committed: https://code.google.com/p/dart/source/detail?r=17861

[Bob Nystrom, 2013-01-30 00:24:05]

This fixes the bug I was seeing where two tests were interfering with each
other. There were a couple of problems:

1. The clean-up code wasn't running if a test failed.
2. When the error was reported, that caused unittest to synchronously
immediately start running the next test before the current one had finished
shutting itself down.

This uses the new Future API and is, I think, cleaner. Please review it
carefully though since this is so easy to get wrong. Thanks!

[nweiz, 2013-01-30 00:44:06]

lgtm