src/vs/base/browser/ui/tree/dataTree.ts/DataTree/updateChildren
class DataTree:	updateChildren(element: TInput | T = this.input!): void {
		if (typeof this.input === 'undefined') {
			throw new TreeError(this.user, 'Tree input not set');
		}

		let isCollapsed: ((el: T) => boolean | undefined) | undefined;

		if (this.identityProvider) {
			isCollapsed = element => {
				const id = this.identityProvider!.getId(element).toString();
				const node = this.nodesByIdentity.get(id);

				if (!node) {
					return undefined;
				}

				return node.collapsed;
			};
		}

		this._refresh(element, isCollapsed);
	}