Migrate to Dart 3 (#210)

Fix https://github.com/google/file.dart/issues/209

I also made sure `dart analyzer` result is perfect.

`abstract class` => `mixin`. They can't be `mixin class` because then they need to expose/re-implement dozens of methods.

![image](https://user-images.githubusercontent.com/351125/226994562-5d1f8653-a358-464b-b201-4edafdc47525.png)
12 files changed
tree: 99bd6be7e35b2c21374bcf04c2a2195436ca5641
  1. .github/
  2. packages/
  3. .gitignore
  4. README.md
README.md

Dart

A monorepo for the package:file and package:file_testing packages.

PackageDescriptionPublished Version
fileA pluggable, mockable file system abstraction for Dart.pub package
file_testingTesting utilities for package:file.pub package