Verify that TFA inferred types are as specific as Dart types

[osa1]

It would be helpful, for catching any bugs in TFA's inferred types, if we could verify that TFA always infers more precise types than the Dart types of analyzed members.

For example, if the Dart return type of a member is Object, it's valid to infer the actual return type of it as int, but it's invalid (a bug in TFA) to infer it as Object?.

This would catch bugs like flutter/flutter#148617 (comment), and maybe also help improve inferred types in cases like #53921 (comment).

cc @alexmarkov