Duration rounding reaches unreachable code

```javascript
import { Temporal } from 'proposal-temporal';
let d = Temporal.Duration.from({ minutes: 10, seconds: 52 });
d.round({ smallestUnit: 'minutes' });
```
leads to
```
        throw new Error('assert not reached');
              ^
Error: assert not reached
    at Object.BalanceDuration (...)
    at Duration.round (...)
```
Happens on latest main. Probably shouldn't?