Now that #97848 is fixed, we should sweep all of our plugins for any nullable generics that were nullable only because of that issue (that's probably any nullable generic, but there may be some rare exceptions). Many have a TODO annotated with #97848, but we shouldn't assume they all have an annotation.

In general I would expect that we can just:

• Remove the nullability from generics.
• Update to the latest version of Pigeon.
• Remove any casting on the Dart side.
• Do any incidental changes for intervening Pigeon breaking changes (e.g., setup->setUp)

Since most of our plugins are using Obj-C and Java on the native side still, native changes will likely be minimal.

This will give us a nearly-free improvement in type safety, and also avoid having other developers who look to our plugins as examples thinking they still need the same workaround.