FileSystem `root` is a DirectoryEntry.

This is specified in the official standard:
http://www.w3.org/TR/file-system-api/#the-filesystem-interface

Interestingly enough, these typings already specify the `root` of
FileSystemSync (the synchronous version of the API) to be a DirectorySync. This must have been an oversight of the original author of these typings.
Merge pull request #1909 from jvilk/filesystem-fixes

filesystem: `root` is a DirectoryEntry.