Replace DiagnosticableMixin with DiagnosticableTreeMixin

[TetsuFe]

To Fix #123

ref

#118 (comment)

flutter doctor

$ flutter doctor
Doctor summary (to see all details, run flutter doctor -v):
[✓] Flutter (Channel stable, 1.20.1, on Mac OS X 10.15.6 19G73, locale ja-JP)

[✓] Android toolchain - develop for Android devices (Android SDK version 28.0.3)
[✓] Xcode - develop for iOS and macOS (Xcode 11.6)
[✓] Android Studio (version 3.6)
[✓] VS Code (version 1.47.3)
[✓] Connected device (1 available)

• No issues found!

[imshashank]

Please merge this soon.

[imshashank]

Anyone having same issue can use this->

  flutter_cupertino_date_picker:
    git:
      url: https://github.com/imshashank/flutter-cupertino-date-picker # Repo
      ref: master # Branch name

[LugonjaAleksandar]

@dylanwuzh can someone from the team who is maintaining this repo please review this? Your repo is being redirected to a fork that is going around the issue with latest Flutter. You guys need to move forward the project by supporting latest breaking changes in Flutter.

[stevenspiel]

It doesn't look like this repo is being maintained. The last commit was 5 months ago.

[elvisun]

Is anyone interested in forking this repo and submitting a new package to pub?...