Adds `ObservableValue` class for observing state changes.

[lrhn]

@macjohnny

[natebosch]

Overall I'm worried about putting this interface into package:async from the start. I'd feel better experimenting in some other place - either internally or in a separate package - so that we can iterate on the API. That was ultimately the conclusion of https://github.com/dart-lang/async/issues/31, but then we never made the push to land something. If we have more motivation now I'm in favor of continuing the exploration.

[lrhn]

If we are going to do more than just this class, we should probably do it in a separate package.

But, if RxDart exists, and I'm sure there are other "observable" frameworks too, do we need to? How would our code distinguish itself from existing code? (Not going to complete on completeness, and "better code quality" can be achieved by contributing to the existing projects, if they need it at all, and if it's going to be a "different API", then it doesn't match that people are always asking for something similar. Not that I don't have ... opinions ... on the Rx APIs.)

[mosuem]

Closing this, as it is stale and the files are out of date. Feel free to reopen in case you still want to land it!