Limit the length of file paths used in many_files_test.

Windows and OS X have pretty strict limits on how long a path can be.

R=rnystrom@google.com
BUG= dartbug.com/22878

Review URL: https://codereview.chromium.org//1013753015

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@44676 260f80e4-7a28-3924-810f-c04153c831b5