Fix spec-compliance bug in Array.prototype.join.

src/js/array.js

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 (function(global, utils, extrasUtils) {
 
 "use strict";
 
 %CheckIsBootstrapping();
 
@@ skipping 196 matching lines @@
         var e = array[i];
         if (!IS_STRING(e)) e = convert(e);
         elements[elements_length++] = e;
       }
       elements.length = elements_length;
       return %StringBuilderConcat(elements, elements_length, '');
     }
     // Non-empty separator case.
     // If the first element is a number then use the heuristic that the
     // remaining elements are also likely to be numbers.
-    if (!IS_NUMBER(array[0])) {
-      for (var i = 0; i < length; i++) {
-        var e = array[i];
+    var e = array[0];
+    if (!IS_NUMBER(e)) {
+      if (!IS_STRING(e)) e = convert(e);
+      elements[0] = e;
+      for (var i = 1; i < length; i++) {
+        e = array[i];
         if (!IS_STRING(e)) e = convert(e);
         elements[i] = e;
       }
     } else {
-      for (var i = 0; i < length; i++) {
-        var e = array[i];
+      elements[0] = %_NumberToString(e);
+      for (var i = 1; i < length; i++) {
+        e = array[i];
         if (IS_NUMBER(e)) {
           e = %_NumberToString(e);
         } else if (!IS_STRING(e)) {
           e = convert(e);
         }
         elements[i] = e;
       }
     }
     return %StringBuilderJoin(elements, length, separator);
   } finally {
@@ skipping 1724 matching lines @@
 %InstallToContext([
   "array_pop", ArrayPop,
   "array_push", ArrayPush,
   "array_shift", ArrayShift,
   "array_splice", ArraySplice,
   "array_slice", ArraySlice,
   "array_unshift", ArrayUnshift,
 ]);
 
 });