Fix the Unit Testing article re: Async

I've updated the [Asynchronous tests](https://www.dartlang.org/articles/dart-unit-tests/#asynchronous-tests) section of the Unit Testing with Dart article with the following:

* [x] code format a few function names in text.
* [x] fix references to `expectAsync0`, `expectAsyncN`, and `expectAsyncUntilN` since they were renamed ([announcement](https://groups.google.com/a/dartlang.org/forum/#!topic/misc/AWCX_ZjDxw4)).
* [x] entirely delete references to `protectAsyncN` and `guardAsync` as they were removed (see above announcement).
* [ ] add new text regarding how exceptions are handled in callbacks to replace the `protectAsyncN` paragraph.
* [ ] add links to the [unittest API documentation](https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/unittest/unittest). But is that not something we do in articles?