src/vs/platform/contextkey/browser/contextKeyService.ts/AbstractContextKeyService/createOverlay
class AbstractContextKeyService:	createOverlay(overlay: Iterable<[string, any]> = Iterable.empty()): IContextKeyService {
		if (this._isDisposed) {
			throw new Error(`AbstractContextKeyService has been disposed`);
		}
		return new OverlayContextKeyService(this, overlay);
	}