dart2js: Support generalized void

[eernstg]

This is the dart2js 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 dart2js 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 dart2js at this point.

[kmillikin]

This should be supported in dart2js now as of 9bdf248 which enabled support in the CFE.