Missing symbols in static Flutter plugin builds

When adding some new usage of `.listener` for blocks, I started hitting this error at runtime:

>Unhandled Exception: Invalid argument(s): Couldn't resolve native function 'disposeObjCBlockWithClosure' in 'objective_c.framework/objective_c' : No asset with id 'objective_c.framework/objective_c' found. No available native assets. Attempted to fallback to process lookup. dlsym(RTLD_DEFAULT, disposeObjCBlockWithClosure): symbol not found.

It looks like there's nothing guaranteeing that this symbol is preserved, so in certain build configurations it's being dead-code stripped. For some important context here, CocoaPod-based plugins can be built in two configurations: as frameworks that are bundled into the app bundle, or as static libraries that are linked directly into the Runner. This depends on both plugin-level settings and app-level settings. `flutter create` using a Swift app template defaults to [forcing framework builds](https://github.com/flutter/flutter/blob/a782869b342953ef15769999cbdc5f3caf5de0d8/packages/flutter_tools/templates/cocoapods/Podfile-ios-swift#L31) (for somewhat complicated and mostly legacy reasons), while an Obj-C app template [does not](https://github.com/flutter/flutter/blob/a782869b342953ef15769999cbdc5f3caf5de0d8/packages/flutter_tools/templates/cocoapods/Podfile-ios-objc#L30). In the new SPM-based system Flutter is moving to, there is a similar divergence in build types depending on whether it's a release build.

I'm currently working on a plugin old enough that its example app is Obj-C, so it uses static library builds. In that app, `disposeObjCBlockWithClosure` is not present anywhere in my final build; the plugin is being built statically and linked in, but the symbol isn't in the resulting binary, so presumably it's been stripped. I was able to fix my app by adding `use_frameworks!`, but plugins need to support both modes.

I assume this will be migrating to native assets at some point; I'm not sure how far out that is, and if we need another workaround in the short term.