Always print an exit code explicitly (no return of any exit code was causing the

Always print an exit code explicitly (no return of any exit code was causing the
Windows bots to hang).
Committed: https://code.google.com/p/dart/source/detail?r=13368

[Emily Fortuna, 2012-10-08 18:49:53]

This changes test_progress.dart to always explicitly return a return code as it
was doing previously:
http://code.google.com/p/dart/source/diff?spec=svn13367&r=13286&format=side&p...

The windows bots are hanging, and it appears from the logs it is because no exit
code is being returned. This is a test to see if this fixes it. I'll revert this
if this is not in fact the issue.

[gram, 2012-10-08 18:51:31]

lgtm