src/vs/workbench/api/common/extHostTreeViews.ts/ExtHostTreeView/getChildren
class ExtHostTreeView:	getChildren(parentHandle: TreeItemHandle | Root): Promise<ITreeItem[]> {
		const parentElement = parentHandle ? this.getExtensionElement(parentHandle) : undefined;
		if (parentHandle && !parentElement) {
			this.logService.error(`No tree item with id \'${parentHandle}\' found.`);
			return Promise.resolve([]);
		}

		const childrenNodes = this.getChildrenNodes(parentHandle); // Get it from cache
		return (childrenNodes ? Promise.resolve(childrenNodes) : this.fetchChildrenNodes(parentElement))
			.then(nodes => nodes.map(n => n.item));
	}