Always send kill signal to debug target

Make sure we kill the debug target process in all cases
to ensure that no zombie processes linger around.

Also make sure the cleanup code gets only called once.

Review URL: https://codereview.chromium.org//15295014

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@22866 260f80e4-7a28-3924-810f-c04153c831b5
1 file changed