Using `FileSystem` with `IOOverrides` from `dart:io`

[jonasfj]

package:file provides a nice abstract implementation of the file-system interfaces from dart:io. But it doesn't supply a means of running existing code (that uses dart:io) with an IOOverrides that delegates to the FileSystem.

I think an implementation might be as simple as:

import 'dart:io';

import 'package:file/file.dart' as f;

/// Creates an [IOOverrides] that uses [fs] for all operations.
IOOverrides createFileSystemIOOverrides(f.FileSystem fs) =>
    _FileSystemIOOverrides(fs);

/// An [IOOverrides] that uses a [f.FileSystem] for all operations.
final class _FileSystemIOOverrides extends IOOverrides {
  final f.FileSystem _fs;

  _FileSystemIOOverrides(this._fs);

  @override
  File createFile(String path) => _fs.file(path);

  @override
  Directory createDirectory(String path) => _fs.directory(path);

  @override
  Link createLink(String path) => _fs.link(path);

  @override
  Future<FileStat> stat(String path) => _fs.stat(path);

  @override
  FileStat statSync(String path) => _fs.statSync(path);

  @override
  Future<bool> fseIdentical(String path1, String path2) =>
      _fs.identical(path1, path2);

  @override
  bool fseIdenticalSync(String path1, String path2) =>
      _fs.identicalSync(path1, path2);

  @override
  Future<FileSystemEntityType> fseGetType(String path, bool followLinks) =>
      _fs.type(path, followLinks: followLinks);

  @override
  FileSystemEntityType fseGetTypeSync(String path, bool followLinks) =>
      _fs.typeSync(path, followLinks: followLinks);

  @override
  Directory getCurrentDirectory() => _fs.currentDirectory;

  @override
  void setCurrentDirectory(String path) {
    _fs.currentDirectory = path;
  }

  @override
  Directory getSystemTempDirectory() => _fs.systemTempDirectory;
}

Maybe, we could add a static FileSystem.runWithIOOverrides(FileSystem fs, ...) that wraps fs in as illustrated above and calls IOOverrides.runWithIOOverrides

We could also just do: abstract base class FileSystem extends IOOverrides, it's a bit of a breaking change. But entirely possible to fix the rest of the package around this.

It's also possible that we shouldn't put this logic into package:file. Because, this packages is about passing in a FileSystem instead of relying on IOOverrides.

[lrhn]

If you add that to the main file_system import, the then it depends on dart:io. Makes it harder to use as an abstraction layer.

Should probably be a separate library.
(Or could be backed by a conditional import, but that'd make it throw at runtime if not running on native.)