Benchmarks for different coding patterns using dart:ffi

Similar to https://github.com/dart-lang/native/issues/718 - it would be nice to have benchmarks to consult for various some non-trivial yet pretty common cases.

For example, what is the performance of an FFI call that returns a fixed size map (a couple of items, some layers) compared to method channels. This should be implemented in all languages, i.e. for Objective-C, that means converting from NSDictionary (or accessing items), to actually compare that to a real-world API that returns a dictionary as its output.

The jnigen issue linked above gives some thoughts on how these benchmarks could be implemented & integrated.