How to use the @theia/filesystem/lib/browser.DirNode.is function in @theia/filesystem

protected updateSelectionContextKeys(): void {
        this.contextKeyService.explorerResourceIsFolder.set(DirNode.is(this.model.selectedNodes[0]));
    }

export function is(node: Object | undefined): node is WorkspaceRootNode {
        return DirNode.is(node) && WorkspaceNode.is(node.parent);
    }

protected getClassNames(element: URI | URIIconReference | FileStat | FileStatNode | WorkspaceRootNode): string[] {
        if (WorkspaceRootNode.is(element)) {
            const name = this.labelProvider.getName(element);
            if (element.expanded) {
                return [this.rootFolderExpandedIcon, this.expandedFolderNameIcon(name)];
            }
            return [this.rootFolderIcon, this.folderNameIcon(name)];
        }
        if (DirNode.is(element)) {
            if (element.expanded) {
                const name = this.labelProvider.getName(element);
                return [this.folderExpandedIcon, this.expandedFolderNameIcon(name)];
            }
            return this.getFolderClassNames(element);
        }
        if (FileStatNode.is(element)) {
            return this.getFileClassNames(element, element.fileStat.uri);
        }
        if (FileStat.is(element)) {
            if (element.isDirectory) {
                return this.getFolderClassNames(element);
            }
            return this.getFileClassNames(element, element.uri);
        }
        if (URIIconReference.is(element)) {