[ddc] Generator functions element type soundness issue

[eernstg]

This is the DDC specific issue for #54159, which has the details.

[sigmundch]

FYI - a new API was added to the CFE to make this easy to implement on the backends. See for example https://dart-review.googlesource.com/c/sdk/+/341397

[eernstg]

As far as I can see, all configurations including all DDC configurations succeed when running the tests mentioned in #54159.

@nshahan, I think this issue can be closed at this time, do you agree?

[nshahan]

Thanks! I don't remember us implementing anything specific here, but if there are no failing tests to illustrate a deficiency I'm happy to close the issue.