LibJS: Remove faulty assertion in BalanceDurationRelative

This is an editorial change in the Temporal spec.

See: https://github.com/tc39/proposal-temporal/commit/66f7464

Userland/Libraries/LibJS/Runtime/Temporal/Duration.cpp

@@ -783,8 +783,11 @@ ThrowCompletionOr<DateDurationRecord> balance_duration_relative(GlobalObject& gl
         return create_date_duration_record(years, months, weeks, days);
     }
 
-    // 2. Assert: relativeTo is not undefined, because callers of this operation ensure relativeTo is required in conditions where this algorithm does not return in step 1.a.
-    VERIFY(!relative_to_value.is_undefined());
+    // 2. If relativeTo is undefined, then
+    if (relative_to_value.is_undefined()) {
+        // a. Throw a RangeError exception.
+        return vm.throw_completion<RangeError>(global_object, ErrorType::TemporalMissingStartingPoint, "calendar units");
+    }
 
     // 3. Let sign be ! DurationSign(years, months, weeks, days, 0, 0, 0, 0, 0, 0).
     auto sign = duration_sign(years, months, weeks, days, 0, 0, 0, 0, 0, 0);

Userland/Libraries/LibJS/Tests/builtins/Temporal/Duration/Duration.prototype.round.js

@@ -161,4 +161,16 @@ describe("errors", () => {
             "A starting point is required for balancing calendar units"
         );
     });
+
+    // Spec Issue: https://github.com/tc39/proposal-temporal/issues/2124
+    // Spec Fix: https://github.com/tc39/proposal-temporal/commit/66f7464aaec64d3cd21fb2ec37f6502743b9a730
+    test("balancing calendar units with largestUnit set to 'year' and relativeTo unset throws instead of crashing", () => {
+        const duration = new Temporal.Duration(1);
+        expect(() => {
+            duration.round({ largestUnit: "year" });
+        }).toThrowWithMessage(
+            RangeError,
+            "A starting point is required for balancing calendar units"
+        );
+    });
 });