[intl] fix off-by-1 in NumberFormat formatToParts parameter parsing

R=adamk@chromium.org, mstarzinger@chromium.org

Bug: v8:5244, chromium:765479
Cq-Include-Trybots: master.tryserver.v8:v8_linux_noi18n_rel_ng
Change-Id: I684805acc194a93b96d74e3e64834867dce78dee
Reviewed-on: https://chromium-review.googlesource.com/668677Reviewed-by: Daniel Ehrenberg <littledan@chromium.org>
Commit-Queue: Josh Wolfe <jwolfe@igalia.com>
Cr-Commit-Position: refs/heads/master@{#48069}

src/builtins/builtins-intl.cc

@@ -351,7 +351,7 @@ BUILTIN(NumberFormatPrototypeFormatToParts) {
   }
 
   Handle<Object> x;
-  if (args.length() >= 1) {
+  if (args.length() >= 2) {
     ASSIGN_RETURN_FAILURE_ON_EXCEPTION(isolate, x,
                                        Object::ToNumber(args.at(1)));
   } else {

test/test262/local-tests/test/intl402/NumberFormat/prototype/formatToParts/default-parameter.js

+// Copyright (C) 2017 Josh Wolfe. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+/*---
+esid: #sec-intl.numberformat.prototype.formattoparts
+description: Intl.NumberFormat.prototype.formatToParts called with no parameters
+info: >
+  Intl.NumberFormat.prototype.formatToParts ([ value ])
+
+  3. If value is not provided, let value be undefined.
+---*/
+
+var nf = new Intl.NumberFormat();
+
+// Example value: [{"type":"nan","value":"NaN"}]
+var implicit = nf.formatToParts();
+var explicit = nf.formatToParts(undefined);
+
+assert(partsEquals(implicit, explicit),
+  "formatToParts() should be equivalent to formatToParts(undefined)");
+
+function partsEquals(parts1, parts2) {
+  if (parts1.length !== parts2.length) return false;
+  for (var i = 0; i < parts1.length; i++) {
+    var part1 = parts1[i];
+    var part2 = parts2[i];
+    if (part1.type !== part2.type) return false;
+    if (part1.value !== part2.value) return false;
+  }
+  return true;
+}