Update the spec for PlainDate.add/subtract. (#1269)

* Update the spec for PlainDate.add/subtract.

Fixes #1251.

* Address code review comment

dateSubtract no longer exists.

Co-authored-by: Philip Chimento <pchimento@igalia.com>

spec/plaindate.html

@@ -371,9 +371,11 @@ <h1>Temporal.PlainDate.prototype.add ( _temporalDurationLike_ [ , _options_ ] )<
         1. Let _balanceResult_ be ? BalanceDuration(_duration_.[[Days]], _duration_.[[Hours]], _duration_.[[Minutes]], _duration_.[[Seconds]], _duration_.[[Milliseconds]], _duration_.[[Microseconds]], _duration_.[[Nanoseconds]], *"days"*).
         1. Set _options_ to ? NormalizeOptionsObject(_options_).
         1. Let _overflow_ be ? ToTemporalOverflow(_options_).
-        1. Let _result_ be ? AddDate(_temporalDate_.[[ISOYear]], _temporalDate_.[[ISOMonth]], _temporalDate_.[[ISODay]], _duration_.[[Years]], _duration_.[[Months]], _duration_.[[Weeks]], _balanceResult_.[[Days]], _overflow_).
-        1. Assert: ! ValidateDate(_result_.[[Year]], _result_.[[Month]], _result_.[[Day]]) is *true*.
-        1. Return ? CreateTemporalDateFromInstance(_temporalDate_, _result_.[[Year]], _result_.[[Month]], _result_.[[Day]]).
+        1. Let _dateAddOptions_ be ! OrdinaryObjectCreate(%Object.prototype%).
+        1. Perform ! CreateDataPropertyOrThrow(_dateAddOptions_, *"overflow"*, _overflow_).
+        1. Let _constructor_ be ? SpeciesConstructor(_temporalDate_, %Temporal.PlainDate%).
+        1. Let _dateAdd_ be ? Get(_calendar_, *"dateAdd"*).
+        1. Return ? Call(_dateAdd_, _calendar_, « _temporalDate_, _duration_, _dateAddOptions_, _constructor_ »).
       </emu-alg>
     </emu-clause>
 
@@ -391,9 +393,12 @@ <h1>Temporal.PlainDate.prototype.subtract ( _temporalDurationLike_ [ , _options_
         1. Let _balanceResult_ be ? BalanceDuration(_duration_.[[Days]], _duration_.[[Hours]], _duration_.[[Minutes]], _duration_.[[Seconds]], _duration_.[[Milliseconds]], _duration_.[[Microseconds]], _duration_.[[Nanoseconds]], *"days"*).
         1. Set _options_ to ? NormalizeOptionsObject(_options_).
         1. Let _overflow_ be ? ToTemporalOverflow(_options_).
-        1. Let _result_ be ? SubtractDate(_temporalDate_.[[ISOYear]], _temporalDate_.[[ISOMonth]], _temporalDate_.[[ISODay]], _duration_.[[Years]], _duration_.[[Months]], _duration_.[[Weeks]], _balanceResult_.[[Days]], _overflow_).
-        1. Assert: ! ValidateDate(_result_.[[Year]], _result_.[[Month]], _result_.[[Day]]) is *true*.
-        1. Return ? CreateTemporalDateFromInstance(_temporalDate_, _result_.[[Year]], _result_.[[Month]], _result_.[[Day]]).
+        1. Let _negatedDuration_ be ? CreateTemporalDuration(−_duration_.[[Years]], −_duration_.[[Months]], −_duration_.[[Weeks]], −_duration_.[[Days]], −_duration_.[[Hours]], −_duration_.[[Minutes]], −_duration_.[[Seconds]], −_duration_.[[Milliseconds]], −_duration_.[[Microseconds]], −_duration_.[[Nanoseconds]]).
+        1. Let _dateAddOptions_ be ! OrdinaryObjectCreate(%Object.prototype%).
+        1. Perform ! CreateDataPropertyOrThrow(_dateAddOptions_, *"overflow"*, _overflow_).
+        1. Let _constructor_ be ? SpeciesConstructor(_temporalDate_, %Temporal.PlainDate%).
+        1. Let _dateAdd_ be ? Get(_calendar_, *"dateAdd"*).
+        1. Return ? Call(_dateAdd_, _calendar_, « _temporalDate_, _negatedDuration_, _dateAddOptions_, _constructor_ »).
       </emu-alg>
     </emu-clause>