WIP: [objective_c] Experimenting with isolate group shared

[liamappelbe]

Based on native assets migration, so that tests can be run in pure Dart: #2329

I'm currently trying to get block_test.dart to pass. To experiment with it yourself, check out this branch, navigate to the pkgs/ffigen directory, then run:

dart pub upgrade
dart test/setup.dart
dart run ffigen --config test/native_objc_test/block_config.yaml
dart --experimental-shared-data test/native_objc_test/block_test.dart

After that you can just run that last command to rerun the test. If you manually edit the bindings (in block_bindings.dart) make sure not to rerun the ffigen command, because your changes will be overwritten.

Most of the test is commented out. The one I'm working on is 'Listener block same thread'. It passes, but the test doesn't terminate. Not sure why yet.