Temporal: Test observable calendar.yearMonthFromFields() calls with null-prototype options

As of https://github.com/tc39/proposal-temporal/pull/2219 PlainYearMonth's
add() and subtract() methods should be calling the calendar's
yearMonthFromFields() method with a null-prototype object as the options
parameter, due to the change in
AddDurationToOrSubtractDurationFromPlainYearMonth. This adds a test for
this behaviour.

harness/temporalHelpers.js

@@ -903,6 +903,7 @@ var TemporalHelpers = {
     class CalendarCheckOptionsPrototypePollution extends Temporal.Calendar {
       constructor() {
         super("iso8601");
+        this.yearMonthFromFieldsCallCount = 0;
         this.dateUntilCallCount = 0;
       }
 
@@ -910,6 +911,12 @@ var TemporalHelpers = {
         return "options-null-proto";
       }
 
+      yearMonthFromFields(fields, options) {
+        this.yearMonthFromFieldsCallCount++;
+        assert.sameValue(Object.getPrototypeOf(options), null, "yearMonthFromFields should be called with null-prototype options");
+        return super.yearMonthFromFields(fields, options);
+      }
+
       dateUntil(one, two, options) {
         this.dateUntilCallCount++;
         assert.sameValue(Object.getPrototypeOf(options), null, "dateUntil should be called with null-prototype options");

test/built-ins/Temporal/PlainYearMonth/prototype/add/calendar-yearmonthfromfields-called-with-null-prototype-options.js

@@ -0,0 +1,17 @@
+// Copyright (C) 2022 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plainyearmonth.prototype.add
+description: >
+    Calendar.yearMonthFromFields method is called with a null-prototype object
+    as the options value when call originates internally
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const calendar = TemporalHelpers.calendarCheckOptionsPrototypePollution();
+const instance = new Temporal.PlainYearMonth(2019, 6, calendar);
+const argument = new Temporal.Duration(1, 1);
+instance.add(argument);
+assert.sameValue(calendar.yearMonthFromFieldsCallCount, 1, "yearMonthFromFields should be called on the calendar");

test/built-ins/Temporal/PlainYearMonth/prototype/subtract/calendar-yearmonthfromfields-called-with-null-prototype-options.js

@@ -0,0 +1,17 @@
+// Copyright (C) 2022 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plainyearmonth.prototype.subtract
+description: >
+    Calendar.yearMonthFromFields method is called with a null-prototype object
+    as the options value when call originates internally
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const calendar = TemporalHelpers.calendarCheckOptionsPrototypePollution();
+const instance = new Temporal.PlainYearMonth(2019, 6, calendar);
+const argument = new Temporal.Duration(1, 1);
+instance.subtract(argument);
+assert.sameValue(calendar.yearMonthFromFieldsCallCount, 1, "yearMonthFromFields should be called on the calendar");