Switch package:objective_c from a flutter plugin to using native assets

[liamappelbe]

This will be possible once native assets doesn't require an experimental flag.

• [x] Remove all the flutter infra and deps from package:objective_c
• [x] Add build.dart infra instead
• [ ] package:ffigen's objc examples were excluded from bot analysis. Include them again, and update them to remove calls to package:objective_c's setup script.
• [x] Throughout package:ffigen and package:objective_c's ObjC tests, we're loading symbols by doing DynamicLibrary.open(...) to load the symbols into the process, then relying on @Native's process loading fallback to find the symbol. Remove this.
• [x] Define a fallback case for the release dispatcher, when running on the standalone embedder. GC tests should pass in both flutter test and dart test. See #1470