[ffigen] Don't generate block helpers for package:objective_c

[liamappelbe]

There's no reason to include block helpers in package:objective_c, as they're not exported publicly. If users need to interact with an API in package:objective_c that uses blocks, they use the helpers code genned in their own bindings. The native trampoline functions for these blocks are compiled into the package:objective_c dylib, which also causes a bit of code bloat. We just need to look for the package:objective_c config flag, and then not code gen the block helpers if we see this flag.

[Eshaan-byte]

Hello @liamappelbe can I work on this issue? Thank you?

[liamappelbe]

@Eshaan-byte sure, go for it. The special config option that package:objective_c sets is here. The block codegen happens in objc_block.dart, so just check the config flag and don't generate the dart code or native code for the blocks if it's true.

[liamappelbe]

@Eshaan-byte Also, bonus points if you measure the package:objective_c dylib size before and after, to see how much bloat this removes