* Make `FileSystem.isLink` actually work

`FileSystem.isLink` called `FileSystem.type` with the default
argument of `followLinks: true`.  This is obviously wrong since it
would mean that `isLink` would never return true (except maybe for
broken symbolic links).

Make `FileSystem.isLink` use `followLinks: false`.  This is what
`dart:io`'s `FileSystemEntity.isLink` is documented to do.

Partially addresses https://github.com/google/file.dart/issues/8.

* Reorganize isFile/isDirectory/isLink tests

Reorganize the isFile/isDirectory/isLink tests so that the test names
are more appropriate.

* Fix MemoryFileSystem to treat an empty path as non-existent

`_MemoryFileSystem.findNode` returns `reference?.directory` if given
the root path or an empty path.  However, an empty path should
be treated as non-existent, not as a directory.  Instead explicitly
return `null` as a special case when given an empty path.

Partially addresses https://github.com/google/file.dart/issues/198.

* Make _MemoryFileSystem._current nullable

Route stream errors to the stream completer rather than the file completer.

Fixes #219

…int>> again (#217)

* Make MemoryFile.openRead and _ChrootFile.openRead return Stream<List<int>> again

This is equivalent to #168,
but I foolishly neglected to make corresponding changes for all
file system types.

Fixes https://github.com/google/file.dart/issues/193.

* Apply suggestions from natebosch

Co-authored-by: Nate Bosch <[email protected]>

---------

Co-authored-by: Nate Bosch <[email protected]>