Analyzer: Implementation of super-bounded types with null safety

[eernstg]

With null safety, the spec update in dart-lang/language#1133 calls for the use of Never rather than Null during instantiation to bound, and during the checks for being a correctly super-bounded type.

Some work was done on this in 744e34a, but the following example illustrates that the analyzer still uses Null:

class A<X extends void Function(X)> {}

void main() {
  A a = A<Never>();
}

The response (with the analyzer from commit ecc87a1) is as follows:

ERROR|COMPILE_TIME_ERROR|TYPE_ARGUMENT_NOT_MATCHING_BOUNDS|/usr/local/google/home/eernst/lang/dart/scratch/202009/n025.dart|4|3|1|'void Function(Null)' doesn't extend 'void Function(void Function(Null))'.

This shows that the raw A is expanded by i2b to A<void Function(Null)>, and then the bounds check fails.

When A is expanded to A<void Function(Never)> the type is correctly super-bounded and the raw A is allowed, and this is the expected outcome after #1133.

[scheglov]

https://dart-review.googlesource.com/c/sdk/+/164860