Bug 1403962 - Add JS test for FillWithUndefined special-case. r=anba

MozReview-Commit-ID: FBa8qw3kUSU

--HG--
extra : rebase_source : 170c3c691ad4b006e0895590d89ff1220e930c13
extra : histedit_source : d0c55eb8a841ca12b59af8bfe7def002a95538be

js/src/jit-test/tests/arrays/fillwithundefined-length-nonwriteable.js

@@ -0,0 +1,16 @@
+var called = false;
+var a = [/* hole */, undefined, {
+    toString() {
+        if (!called) {
+            called = true;
+            a.length = 3;
+            Object.defineProperty(a, "length", {writable:false});
+        }
+        return 0;
+    }
+}, 0];
+a.sort();
+
+assertEq(a.length, 3);
+assertEq(a[1], 0);
+assertEq(a[2], undefined);