Test that getCanonicalLocales() returns a mutable object. (#746)

Will fix #745

Besides, add additional checks to make sure that getCanonicalLocales()
returns an array.

test/intl402/Intl/getCanonicalLocales/returned-object-is-an-array.js

@@ -8,12 +8,14 @@ info: |
   8.2.1 Intl.getCanonicalLocales (locales)
   1. Let ll be ? CanonicalizeLocaleList(locales).
   2. Return CreateArrayFromList(ll).
-includes: [compareArray.js]
 ---*/
 
 var locales = ['en-US'];
 var result = Intl.getCanonicalLocales(['en-US']);
 
+assert.sameValue(Object.getPrototypeOf(result), Array.prototype, 'prototype is Array.prototype');
+assert.sameValue(result.constructor, Array);
+
 assert.notSameValue(result, locales, "result is a new array instance");
 assert.sameValue(result.length, 1, "result.length");
 assert(result.hasOwnProperty("0"), "result an own property `0`");

test/intl402/Intl/getCanonicalLocales/returned-object-is-mutable.js

+// Copyright (C) 2016 the V8 project authors. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+/*---
+esid: sec-intl.getcanonicallocales
+description: >
+  Tests that the value returned by getCanonicalLocales is a mutable array.
+info: |
+  8.2.1 Intl.getCanonicalLocales (locales)
+  1. Let ll be ? CanonicalizeLocaleList(locales).
+  2. Return CreateArrayFromList(ll).
+includes: [propertyHelper.js]
+---*/
+
+var locales = ['en-US', 'fr'];
+var result = Intl.getCanonicalLocales(locales);
+
+verifyEnumerable(result, 0);
+verifyWritable(result, 0);
+verifyConfigurable(result, 0);
+
+result = Intl.getCanonicalLocales(locales);
+verifyEnumerable(result, 1);
+verifyWritable(result, 1);
+verifyConfigurable(result, 1);
+
+result = Intl.getCanonicalLocales(locales);
+verifyNotEnumerable(result, 'length');
+verifyNotConfigurable(result, 'length');
+
+assert.sameValue(result.length, 2);
+result.length = 42;
+assert.sameValue(result.length, 42);
+assert.throws(RangeError, function() {
+  result.length = "Leo";
+}, "a non-numeric value can't be set to result.length");