Fixed tests to reliably clean up temporary files/directories

Fixed tests to reliably clean up temporary files/directories

This should fix issue: http://dartbug.com/8534

Committed: https://code.google.com/p/dart/source/detail?r=18518

[kustermann, 2013-02-14 02:27:37]

After the simplification of RunningProcess, it's responsibilities reduced to:
running a Command and capturing it's output -- in particular it doesn't call the
'completed()' method on TestCase.

This had the effect that the test 'skipping_dart2js_compilations_test' was
broken from that point on (since the testing+cleanup code was in the completed
handler [which wasn't called anymore]).

[ricow1, 2013-02-14 07:20:32]

LGTM