Correctly process watch registration within reactionFns

[caitp]

From dart-archive/angular.dart@d6bc9ab, it is clear that in the existing implementation, there is an issue with registering watches within a listener. This can result in an infinite loop which (in an Angular context) would result in essentially "infinite digest" errors, or in Angular.dart's case "model did not stabilize".

While we don't have TTL within this library, it should be possible to simulate with callback spies incrementing a counter.