Problem: Wrapping functions and forwarding arguments is noisy

It's not possible to write a signature expressing that the return value is a function having the same argument count and types as one of the parameters. This leads to ugly blocks where we define the same function N times with varying argument counts and lots of duplication in generic types and forwarding the arguments.

For example `expectAsync0` through `expectAsync6`: https://github.com/dart-lang/test/blob/ac21330338e0a7ad68fb19058bb92260706dd6cf/pkgs/test_api/lib/src/frontend/expect_async.dart#L295

This gets even more complex with things like optional arguments.