[flutter_local_notifications_linux] Fix build failing on Web

[proninyaroslav]

Web doesn't support FFI, so we need to strip all transitive dependencies at compile time, that using FFI
Fixed #1247

[proninyaroslav]

@MaikuB
I made a compile-time stub for the Linux plugin, I also removed the runtime checking for the integer width, because this duplicates the functionality of the dbus package. Now building for the Web happens without any problems.

[MaikuB]

Thank. To carry on the convo from the issue thread, I probably wasn't clear enough that I also did a conditional import. The difference though is that the stub had empty implementations for all methods. I won't be able to test this PR until later but I would thought that would be needed as difference between this plugin and the connectivity one is that this plugin has platform-specific methods and the overrides take extra platform-specific parameters. Therefore, I would've thought an app is compiling for the web but makes calls directly to the Linux plugin (e.g. https://pub.dev/documentation/flutter_local_notifications_linux/latest/flutter_local_notifications_linux/LinuxFlutterLocalNotificationsPlugin/initialize.html or https://pub.dev/documentation/flutter_local_notifications_linux/latest/flutter_local_notifications_linux/LinuxFlutterLocalNotificationsPlugin/getSystemIdMap.html), it would fail to compile as these won't exist in the stub.

Another thing is I think it'd still be good to re-implement the posix calls as the posix package depends on ffi versions >= 0.1.3 but less < 1.0.0 to avoid potential problems around dependency conflicts with what's baked into the Flutter SDK. There are very few calls to implement that I'd say it's worth doing to remove posix as a dependency

[proninyaroslav]

it would fail to compile as these won't exist in the stub.

Yes, you are right, thanks for the comment. I forgot that the main plugin implementation calls these methods in code. I moved these methods into a separate interface that extends FlutterLocalNotificationsPlatform, and the real implementation and the stub implement it.
In my local repo, I did a revert that disables Linux support and checked the build of the example app for Linux and Web. I have not noticed any problems.

[proninyaroslav]

Another thing is I think it'd still be good to re-implement the posix calls

Yes, I'll push it today.

[proninyaroslav]

@MaikuB
POSIX calls: d4ec745
Method stubs: e69b18d

[MaikuB]

Left some comments on the PR. Let me know what you think