src/vs/workbench/browser/parts/editor/breadcrumbs.ts/BreadcrumbsService/register
class BreadcrumbsService:	register(group: number, widget: BreadcrumbsWidget): IDisposable {
		if (this._map.has(group)) {
			throw new Error(`group (${group}) has already a widget`);
		}
		this._map.set(group, widget);
		return {
			dispose: () => this._map.delete(group)
		};
	}