Why use milliseconds instead of a Duration?  For example:

```dart
new ClockWatcher.atInterval(aSecond * 5);
// vs.
new ClockWatcher.atInterval(5000);
```

Duration is more readable IMO, and since Durations are prevelant in dart, one would often need to call `inMilliseconds` on their duration if using milliseconds here.

(`aSecond` etc. are defined in `quiver.time` as well)