Updated diagnosticable

[arbyazra123]

No description provided.

[arbyazra123]

Updated diagnosticableMixin to diagnosticable

[hcaz]

This works perfectly and should be merged in with a new build on the pub store. Anybody who needs it before then can add to dependency_overrides in their pubspec.yaml

dependency_overrides:
  flutter_cupertino_date_picker: #https://github.com/dylanwuzh/flutter-cupertino-date-picker/pull/136 Updated diagnosticable
    git:
      url: https://github.com/arbyazra123/flutter-cupertino-date-picker.git
      ref: updatedDiagnosticable

[prafgup]

@dylanwuzh Please merge this