consider splitting Observable into interface vs mixin

[jmesserly]

this would allow, e.g. implements Observable or with ChangeNotifier without transitively importing mirrors. I'm not sure this is necessary given that we use the standard https://github.com/dart-lang/smoke solution, and an improved https://github.com/dart-lang/reflectable is on the way, but maybe as a temporary solution for some customers.

[jmesserly]

on second thought, we should definitely do this. The current Observable mixes protected methods and interface methods. That's no good. Thanks to @matanlurey for catching this.

[jmesserly]

also while we're at it, we should add @protected annotations or make sure the documentation is really clear