12.3.4 Type visibility and dynamic binding

Again, this depends on issue ECMA-TC49-TG2/spec#248.

First bullet in 13.3.4:

> A constituent expression of compile-time type `dynamic` is considered to have the type of the actual value that the expression evaluates to at runtime

IIRC, it's a bit more complicated than that, because the calling code may not have access to that type, so dynamic binding to even public members of that type can't happen. Do we need to go into details of that? I understand it went through various iterations within MS, but I'm not sure whether the result is deemed implementation-specific or something that can be nailed down.