I can be more descriptive, but this says *wall-clock* and its called *watchClock*.  Interval could be renamed to be more descriptive.  I'm against adding `align` since users of this are expecting to be called back with respect to the wall time and if they fire
```dart
watchClock(aMinute).first.then(print);
```
And don't see that fire when they look at a clock, then we've broken our promise.