add examples for enums, async, and deferred loading

That comment could be better (it isn't visible in the text, but I'll fix it). This is indeed an analyzer thing (and in the language spec): https://code.google.com/p/dart/issues/detail?id=21719

I describe it in the text like this:

If the e in switch (e) is explicitly typed as an enum, then you’re warned if you don’t handle all of the enum’s values...