src/vs/editor/contrib/gotoSymbol/peek/referencesTree.ts/DataSource/getChildren
class DataSource:	getChildren(element: ReferencesModel | FileReferences | TreeElement): TreeElement[] | Promise<TreeElement[]> {
		if (element instanceof ReferencesModel) {
			return element.groups;
		}

		if (element instanceof FileReferences) {
			return element.resolve(this._resolverService).then(val => {
				// if (element.failure) {
				// 	// refresh the element on failure so that
				// 	// we can update its rendering
				// 	return tree.refresh(element).then(() => val.children);
				// }
				return val.children;
			});
		}

		throw new Error('bad tree');
	}