src/vs/workbench/api/common/extHostCustomEditors.ts/ExtHostCustomEditors/$backup
class ExtHostCustomEditors:	async $backup(resourceComponents: UriComponents, viewType: string, cancellation: CancellationToken): Promise<string> {
		const entry = this.getCustomDocumentEntry(viewType, resourceComponents);
		const provider = this.getCustomEditorProvider(viewType);

		const backup = await provider.backupCustomDocument(entry.document, {
			destination: entry.getNewBackupUri(),
		}, cancellation);
		entry.updateBackup(backup);
		return backup.id;
	}