Darwin build was failing:

    $ ./all.bash
    + go build ./...
    + GOOS=darwin
    + go build ./fuse/... ./fs/... ./example/loopback/...
    # github.com/hanwen/go-fuse/fuse/nodefs
    fuse/nodefs/files.go:99:9: cannot use &loopbackFile{…} (value of type *loopbackFile) as File value in return statement: *loopbackFile does not implement File (wrong type for method Allocate)
                    have Allocate(uint64, uint64, uint32) fuse.Status
                    want Allocate(uint64, uint64, uint32, *fuse.Context) (fuse.Status)
    fuse/nodefs/files_darwin.go:87:22: not enough arguments in call to f.GetAttr
            have (*fuse.Attr)
            want (*fuse.Attr, *fuse.Context)