Add a specific test for relatedYear output. (#2458)

Based on <https://cs.chromium.org/chromium/src/v8/test/intl/date-format/related-year.js>.

Fixes #2332.

test/intl402/DateTimeFormat/prototype/format/related-year-zh.js

@@ -0,0 +1,14 @@
+// Copyright 2019 Google Inc, Igalia S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-partitiondatetimepattern
+description: >
+  Checks the output of 'relatedYear' and 'yearName' type, and
+  the choice of pattern based on calendar.
+locale: [zh-u-ca-chinese]
+---*/
+
+const df = new Intl.DateTimeFormat("zh-u-ca-chinese", {year: "numeric"});
+const date = new Date(2019, 5, 1);
+assert.sameValue(df.format(date), "2019己亥年");

test/intl402/DateTimeFormat/prototype/formatToParts/related-year-zh.js

@@ -0,0 +1,29 @@
+// Copyright 2019 Google Inc, Igalia S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-partitiondatetimepattern
+description: >
+  Checks the output of 'relatedYear' and 'yearName' type, and
+  the choice of pattern based on calendar.
+locale: [zh-u-ca-chinese]
+---*/
+
+function verifyFormatParts(actual, expected, message) {
+  assert.sameValue(Array.isArray(expected), true, `${message}: expected is Array`);
+  assert.sameValue(Array.isArray(actual), true, `${message}: actual is Array`);
+  assert.sameValue(actual.length, expected.length, `${message}: length`);
+
+  for (let i = 0; i < actual.length; ++i) {
+    assert.sameValue(actual[i].type, expected[i].type, `${message}: parts[${i}].type`);
+    assert.sameValue(actual[i].value, expected[i].value, `${message}: parts[${i}].value`);
+  }
+}
+
+const df = new Intl.DateTimeFormat("zh-u-ca-chinese", {year: "numeric"});
+const date = new Date(2019, 5, 1);
+verifyFormatParts(df.formatToParts(date), [
+  {type: "relatedYear", value: "2019"},
+  {type: "yearName", value: "己亥"},
+  {type: "literal", value: "年"},
+]);