The strikes against it are:

1. It does not take path boundaries into account.
2. It assumes that Windows==case-insensitive file system
and non-Windows==case-sensitive file system, neither of
which is always true.
3. Comparing ToLower against ToLower is not a correct
implementation of a case-insensitive string comparison.
4. If it returns true on Windows you still don't know how long
the matching prefix is in bytes, so you can't compute what
the suffix is.

R=golang-dev, r, dsymonds, r
CC=golang-dev
https://golang.org/cl/5712045