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['content'] || additionalProps['version']) {
			throw new Error('Cannot have core properties as additional');
		}

		const version = this.userDataSyncEnablementService.getResourceSyncStateVersion(this.resource);
		const lastSyncUserData = { ref: lastSyncRemoteUserData.ref, content: lastSyncRemoteUserData.syncData ? JSON.stringify(lastSyncRemoteUserData.syncData) : null, version, ...additionalProps };
		await this.fileService.writeFile(this.lastSyncResource, VSBuffer.fromString(JSON.stringify(lastSyncUserData)));
	}