dartdevc: Support generalized void

[eernstg]

This is the dartdevc specific issue for issue #30176, which contains the details.

This issue was created in order to have a suitable issue number to refer to in status files.

The implementation of support for generalized void in dartdevc should be coordinated with that of other tools, and in particular most or all of it may be provided by the common front end. Hence, please do not start implementing this in dartdevc at this point.

[jmesserly]

I don't think we have anything else to do here, FYI. DDC runtime type system supports void (e.g. for function subtyping), and DDC static support will work if Analyzer/CFE support it.