VM: allow void as a type argument

[floitschG]

This is the VM issue for #27504.

[mit-mit]

Punting to 1.24 per dart-leads discussion.

[crelier]

It is not clear to me what feature is required here. It is easy to syntactically allow void as a type argument (see https://codereview.chromium.org/2907423003/), but I need to know how to handle void in type tests (non-strong mode and strong mode). So we need a spec or at least a set of dart tests setting some expectations. Thanks.

[eernstg]

Right, you do of course need that, as do others---and I'll be writing such a spec (in the form of a standalone informal spec, as usual). We're GoogleServing today, but I'll get started writing it tomorrow.