FutureOr, void, and dynamic docs improvements

[jakemac53]

This may not be worth fixing, but in the API docs FutureOr is listed as being a "class", even though it isn't actually one. It is weird though because in the code, it is listed as an abstract class (with docs saying it really isn't a class). So presumably that is why it shows up as a class in dartdoc. This could be confusing, because there really are differences between just being a type and being a class.

In thinking about any other weird types like this, void and dynamic came to mind. Neither of these have any documentation at all that I can find, because we didn't do the abstract class workaround just to get docs.

It might be worth creating a new category called just "type", which we could use just to document these, with some special casing in dartdoc?

[srawlins]

Yeah, I'd love to get that all fixed; the main issue is #2391.

We could special case FutureOr though, to indicate it is not a class.

[jakemac53]

SGTM, feel free to close this as a dupe (originally I searched just for FutureOr issues).