src/vs/base/common/observableImpl/derived.ts/Derived/subscribeTo
class Derived:	public subscribeTo<T>(observable: IObservable<T>) {
		this._dependencies.add(observable);
		// We are already added as observer for stale dependencies.
		if (!this.staleDependencies.delete(observable)) {
			observable.addObserver(this);
		}
	}