src/vs/workbench/api/common/extHostTreeViews.ts/ExtHostTreeView/getChildrenNodes
class ExtHostTreeView:	private getChildrenNodes(parentNodeOrHandle: TreeNode | TreeItemHandle | Root): TreeNode[] | undefined {
		if (parentNodeOrHandle) {
			let parentNode: TreeNode | undefined;
			if (typeof parentNodeOrHandle === 'string') {
				const parentElement = this.getExtensionElement(parentNodeOrHandle);
				parentNode = parentElement ? this.nodes.get(parentElement) : undefined;
			} else {
				parentNode = parentNodeOrHandle;
			}
			return parentNode ? parentNode.children || undefined : undefined;
		}
		return this.roots;
	}