Update DartPluginFile usage for windows

[brianblanchard-wf]

Description

• With the release of Flutter 2.8.0 to the stable channel, there are some changes with how dart only plugin implementations get registered.
• Flutter generates a generated_main.dart file under the .dart-tool/flutter_build directory which for the example app for this plugin looks like this

  
  export 'package:biometric_storage_example/main.dart';

import 'package:biometric_storage_example/main.dart' as entrypoint; import 'dart:io'; // flutter_ignore: dart_io_import. import 'package:biometric_storage/biometric_storage.dart';

@pragma('vm:entry-point') class _PluginRegistrant {

@pragma('vm:entry-point') static void register() { if (Platform.isAndroid) { } else if (Platform.isIOS) { } else if (Platform.isLinux) { } else if (Platform.isMacOS) { } else if (Platform.isWindows) { try { Win32BiometricStoragePlugin.register(); } catch (err) { print( 'biometric_storage threw an error: $err. ' 'The app may not function as expected until you remove this plugin from pubspec.yaml' ); rethrow; }

}

}

}

typedef _UnaryFunction = dynamic Function(List args); typedef _NullaryFunction = dynamic Function();

void main(List args) { if (entrypoint.main is _UnaryFunction) { (entrypoint.main as _UnaryFunction)(args); } else { (entrypoint.main as _NullaryFunction)(); } }

which will cause the following error when building the application

Error: Undefined name 'Win32BiometricStoragePlugin'. .dart_tool/flutter_build/generated_main.dart:38 Win32BiometricStoragePlugin.registerWith();

- The flutter tooling is expecting
  - The `Win32BiometricStoragePlugin` class to be importable from `package:biometric_storage/biometric_storage.dart'` 
  - The `Win32BiometricStoragePlugin` to define a static function `registerWith()` 

## Changes
- Removed the noop win32 file/implementation
- Changed the default `BiometricStorage.instance` to be `MethodChannelBiometricStorage`
- Exported the `Win32BiometricStoragePlugin` from `lib/biometric_storage.dart`
- Defined `Win32BiometricStoragePlugin.registerWith` static method that registers an instance of `Win32BiometricStoragePlugin` as the instance provided by `BiometricStorage.instance`
- Updated some various iOS project files based on Flutter tooling changes
- Update the example to include `biometric_storage` as a normal dependency and not a dev dependency
- Bumped the version to `4.0.0` because this is most likely a breaking change
- Bumped the min Flutter SDK to `2.8.0` because I believe prior to this stable version the issue described above does not reproduce.

## Important Notes
- If you try to reproduce this issue with the current release's example app and Flutter 2.8.0 it will NOT reproduce. The dependency on `biometric_storage` is a dev dependency instead of a normal dependency. It seems the the `generated_main.dart` file is not generated if a `dev_dependency` provides a Dart only plugin implementation. I have updated the example to provide `biometric_storage` as a normal `dependency` in this PR.

## Testing
- I have verified this fixes the build issue on iOS and Android but cannot verify that the Linux and Windows implementations of this plugin are still working as expected because I do not have the proper hardware.

[miDeb]

This does not fix it for me compiling on linux for either android or linux itself:

ERROR: .dart_tool/flutter_build/generated_main.dart:61:9: Error: Undefined name 'Win32BiometricStoragePlugin'.
ERROR:         Win32BiometricStoragePlugin.registerWith();
ERROR:         ^^^^^^^^^^^^^^^^^^^^^^^^^^^

I think this is because pubspec.yaml still references Win32BiometricStoragePlugin. If I remove the reference, the error does not occur.

[brianblanchard-wf]

@miDeb my bad, I forgot to export the Win32BiometricStoragePlugin. Can you try again with the latest commit?

[brianblanchard-wf]

@hpoul as yes, my bad I thought I did that.

[miDeb]

@brianblanchard-wf I can confirm that it works for me now.

[brianblanchard-wf]

@hpoul regarding federated plugins, it might be worth a discussion in breaking out the different platform implementations into different packages similar to how some of the official flutter plugins (like shared_preferences) do it. So you would have something like biometric_storage, biometric_storage_platform_interface, biometric_storage_windows, etc that can all live in this repo as sub packages.

Let me know if that is something that is interesting to you and I can take a stab at it in my free time.

For sure out of scope for this PR, I was just looking to unblock consumers on the latest stable version of Flutter 😄

[brianblanchard-wf]

@hpoul I did a quick test and it seems if I just change the windows plugin entry in the pubspec.yaml to this:

windows:
   pluginClass: none

then no other code changes are required. Do you want to go the route of the current implementation with dartPluginClass or just update the pubspec.yaml to the above?

[hpoul]

@brianblanchard-wf are you saying that the fileName property is working? 🤔️ To me this would look nicer, but since it's not documented, it might be better to go the route of exporting the class from the main library file 😅️ thanks, I'll merge it this way and see if it works for everyone 🤞