No analyzer strong mode error for void sync* and async* functions

[sgrekhov]

Consider the following code

void h() sync* { }

main() {
  h();
}

Alanyzer in nostrong mode produces warning
warning - Functions marked 'sync*' must have a return type assignable to 'Iterable'
I'm expecting an error in a strong mode but there is, surprisingly, no issues!

The same is true for async* functions

void h() async* { }

main() {
  h();
}

warning - Functions marked 'async*' must have a return type assignable to 'Stream'
in a nostrong mode and no issues in a strong one

Tested on dartanalyzer version 2.0.0-dev.26.0 on Windows

[eernstg]

Note that in 9f9987f we updated the feature specification generalized-void.md to specify the following:

It is a static warning (in Dart 2: a compile-time error) if a function marked
`async*` or `sync*` has return type `void`.

So the declarations of h are compile-time errors specifically because they use void as their return type, not because of a missing assignability relationship.

Assignability has changed because of a different change, though, so that's probably the reason why the strong mode analysis does not flag this declaration: void used to be related to dynamic and the bottom type, only, but in Dart 2 it is a top type (like Object and dynamic), and this means that it is assignable to Iterable today, but wasn't previously.

So the Dart 2 static analysis needs to implement the (newly added) specific rule about void, and these situations will then again be flagged as errors.