src/vs/base/common/observableImpl/derived.ts/Derived/onLastObserverRemoved
class Derived:	protected override onLastObserverRemoved(): void {
		/**
		 * We are not tracking changes anymore, thus we have to assume
		 * that our cache is invalid.
		 */
		this.hasValue = false;
		this.hadValue = false;
		this.value = undefined;
		for (const d of this._dependencies) {
			d.removeObserver(this);
		}
		this._dependencies.clear();
	}