src/vs/workbench/api/common/extHostTreeViews.ts/ExtHostTreeViews/$getChildren
class ExtHostTreeViews:	$getChildren(treeViewId: string, treeItemHandle?: string): Promise<ITreeItem[] | undefined> {
		const treeView = this.treeViews.get(treeViewId);
		if (!treeView) {
			return Promise.reject(new Error(localize('treeView.notRegistered', 'No tree view with id \'{0}\' registered.', treeViewId)));
		}
		return treeView.getChildren(treeItemHandle);
	}