Temporal: Add test for custom fields in PlainDate.from.

test/built-ins/Temporal/PlainDate/from/calendar-fields-custom.js

@@ -0,0 +1,44 @@
+// 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.plaindate.from
+description: Verify the result of calendar.fields() is treated correctly.
+includes: [compareArray.js, temporalHelpers.js]
+features: [Temporal]
+---*/
+
+class CustomCalendar extends Temporal.Calendar {
+  constructor() {
+    super("iso8601");
+  }
+  dateFromFields(fields) {
+    assert.compareArray(Object.keys(fields), ["b", "a"]);
+    return new Temporal.PlainDate(2020, 7, 4);
+  }
+  fields(fields) {
+    assert.compareArray(fields, ["day", "month", "monthCode", "year"]);
+    return ["b", "a"];
+  }
+}
+
+const calendar = new CustomCalendar();
+const actual = [];
+const item = new Proxy({ calendar }, {
+  has(target, property) {
+    actual.push(`has item.${property}`);
+    return property in target;
+  },
+  get(target, property) {
+    actual.push(`get item.${property}`);
+    return target[property];
+  },
+});
+
+const plainDate = Temporal.PlainDate.from(item);
+TemporalHelpers.assertPlainDate(plainDate, 2020, 7, "M07", 4);
+assert.compareArray(actual, [
+  "get item.calendar",
+  "get item.b",
+  "get item.a",
+]);