The way custom FlValue objects work on Linux, the ID they use is part of the API contract (to identify them, since there's no inherent runtime typing), but currently those assignments are an implementation detail of the generated Pigeon code. That's an issue for native unit tests, because in order to call fl_value_new_custom_object to make arguments to call handlers, the right ID is required.

This isn't high priority since it can be worked around by hard-coding the IDs in the tests, but that means that tests are subject to breakage when updating the Pigeon generation, which is obviously not ideal. We should make public constants for each of the Pigeon-generated class IDs, so that tests can be written using the constants, and get any updates automatically.