src/vs/platform/undoRedo/common/undoRedoService.ts/UndoRedoService/_invokeWorkspacePrepare
class UndoRedoService:	private async _invokeWorkspacePrepare(element: WorkspaceStackElement): Promise<IDisposable> {
		if (typeof element.actual.prepareUndoRedo === 'undefined') {
			return Disposable.None;
		}
		const result = element.actual.prepareUndoRedo();
		if (typeof result === 'undefined') {
			return Disposable.None;
		}
		return result;
	}