Analyzer error messages don't show '?'s inside function types.

[stereotype441]

Consider the following code:

int? f(int i) => i;
void g(int Function(int?) callback) {}
main() {
  g(f);
}

Running this code through the analyzer with the NNBD experiment enabled produces the following unhelpful error messages:

  error • The argument type 'int Function(int)' can't be assigned to the parameter type 'int Function(int)'. • .../test.dart:4:5 • argument_type_not_assignable
  error • The function 'f' has type 'int Function(int)' that isn't of expected type 'int Function(int)'. This means its parameter or return type doesn't match what is expected. • .../test.dart:4:5 • invalid_cast_function

Setting aside the fact that there are two errors when there really should only be one, the error messages are unhelpful because they don't contain any ?s, so there's no way to see the actual problem.

I believe this bug only manifests when the types that are being printed are function types.

[stereotype441]

@scheglov FYI

[scheglov]

https://dart-review.googlesource.com/c/sdk/+/127744

  error • The argument type 'int? Function(int)' can't be assigned to the parameter type 'int Function(int?)'. • /Users/scheglov/dart/test/bin/test.dart:4:5 • argument_type_not_assignable

The error from checker is not updated (not sure if we need, with DDC on the way out).