Fix windows signal test

(this amends a bit the changes in
https://code.google.com/p/dart/source/detail?r=44902)

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1078803002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@45020 260f80e4-7a28-3924-810f-c04153c831b5