add ValueObservable.combineLatestN + tests

[lukepighetti]

Quick PR just to see what your appetite is for ValueObservable specific methods to preserve ValueObservable type

[brianegan]

I actually really like the idea behind this change. It's something I'd considered in the past as well!

One question about the implementation that I'd love some feedback on: It currently uses shareValueSeeded to achieve the effect. That will mean the Observable will become unusable after the final listener unsubscribes to it. Would that be the expected behavior for peeps using this code?

[lukepighetti]

I was not aware that was a side-effect of using shareValueSeeded. The purpose for using it was to try to preserve value at all costs.

[brianegan]

Heya @lukepighetti -- how are ya feeling about this change?

[lukepighetti]

If you can provide a quick and clear request for the change I'd be happy to implement it and push. My main concern is making sure this is consistent with your vision for rxdart

[brianegan]

One thing I was considering: I don't think most people want shareValueSeeded (from the Rx spec), which will shut down when there are no more listeners. Rather, I think most folks want an operator that works like asBroadcastStream but also captures the latest value. I was thinking of adding two new operators:

asBroadcastValueStream & asBroadcastValueStreamSeeded. These would work exactly like asBroadcastStream -- in fact, they'd use that method under the hood, but would also provide access to the latest emitted value.

Perhaps we could use that base for this change?