I have tested this locally and the tests pass. I hackily checked it for Windows by changing the implementation of isPathSeparator and pathSeparator to be the same as WindowsPlatform's; the tests pass for that too. I can test on my Windows VM as well.

Some things that came up during this fix:

• there might be other methods relying on these bad checks. Checks on path separators should be done using isPathSeparator on either Platform or Char. Directly comparing with thisProcess.platform.pathSeparator is incorrect.
• PathName has no equality comparison operator. IMO, comparing two PathName objects should be the same as comparing their fullPaths
• IMO, we should deprecate withTrailingSlash and withoutTrailingSlash in favor of more appropriate names. This current naming is Unix-biased. I'll open a ticket or PR for it if people agree.

Shoutout to @aspiers - I was using guard for this and it made the whole process so much smoother!

Awesome! Really happy to hear that 😁