Dart's Stream class is missing many of the composition, transformation and termination methods that you'd find in other FRP libraries. It's also lacking a concept of time-varying values, which to me are important for holding and responding to state changes.
Having functions like flatMapLatest, flatMap, combine, merge, scan, and takeUntil are pretty much essential to building any RX heavy application.
There are some packages out there that extend Dart's streams to add this behavior. One of which I'm the author of, Frappe. Of course, it'd be nice if Dart included this stuff out of the box.