This allows listDir to work properly with recursive symlinks on Windows.

BUG=7505
R=rnystrom@google.com

Review URL: https://codereview.chromium.org//14924006

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@22477 260f80e4-7a28-3924-810f-c04153c831b5