src/vs/base/browser/ui/tree/abstractTree.ts/TreeNavigator/constructor
class TreeNavigator:	constructor(private view: ITreeNavigatorView<T, TFilterData>, private model: ITreeModel<T, TFilterData, TRef>, start?: TRef) {
		if (start) {
			this.index = this.model.getListIndex(start);
		} else {
			this.index = -1;
		}
	}