Editorial: Change missing calendar error in RoundDuration to an asser…

…tion

With the previous changes, the throwing step can no longer be reached.

Closes: #2648

polyfill/lib/ecmascript.mjs

@@ -5379,10 +5379,6 @@ export function RoundDuration(
   // dateAdd and dateUntil must be looked up
   const TemporalDuration = GetIntrinsic('%Temporal.Duration%');
 
-  if (IsCalendarUnit(unit) && !plainRelativeTo) {
-    throw new RangeError(`A starting point is required for ${unit}s rounding`);
-  }
-
   // First convert time units up to days, if rounding to days or higher units.
   // If rounding relative to a ZonedDateTime, then some days may not be 24h.
   let dayLengthNs;

spec/duration.html

@@ -2010,7 +2010,7 @@ <h1>
         1. If _zonedRelativeTo_ is not present, set _zonedRelativeTo_ to *undefined*.
         1. If _precalculatedPlainDateTime_ is not present, set _precalculatedPlainDateTime_ to *undefined*.
         1. If IsCalendarUnit(_unit_) is *true*, then
-          1. If _plainRelativeTo_ is *undefined*, throw a *RangeError* exception.
+          1. Assert: _plainRelativeTo_ is not *undefined*.
           1. Assert: CalendarMethodsRecordHasLookedUp(_calendarRec_, ~date-add~) is *true*.
           1. Assert: CalendarMethodsRecordHasLookedUp(_calendarRec_, ~date-until~) is *true*.
         1. If IsCalendarUnit(_unit_) is *true* or _unit_ is *"day"*, then