src/vs/platform/userDataSync/common/abstractSynchronizer.ts/AbstractSynchroniser/updateLastSyncUserData
class AbstractSynchroniser:	protected async updateLastSyncUserData(lastSyncRemoteUserData: IRemoteUserData, additionalProps: IStringDictionary<any> = {}): Promise<void> {
		if (additionalProps['ref'] || additionalProps['version']) {
			throw new Error('Cannot have core properties as additional');
		}

		const version = this.userDataSyncEnablementService.getResourceSyncStateVersion(this.resource);
		const lastSyncUserDataState: ILastSyncUserDataState = {
			ref: lastSyncRemoteUserData.ref,
			version,
			...additionalProps
		};

		this.storageService.store(this.lastSyncUserDataStateKey, JSON.stringify(lastSyncUserDataState), StorageScope.APPLICATION, StorageTarget.MACHINE);
		await this.writeLastSyncStoredRemoteUserData(lastSyncRemoteUserData);
	}