Fix case-insensitive absolute listing.

This goes to show that we shouldn't avoid using path functions in the
name of efficiency. We were using a raw substring to get a relative
path, and it failed for absolute roots because they didn't have an extra
"/" added to the end.

Closes dart-lang/glob#4

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//1773293002 .