Revert "Make toLocaleString on arrays always call toLocaleString on its elements."

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 133 matching lines @@
     if (separator === '') {
       if (keys.length === 0) return '';
       return SparseJoin(array, keys, convert);
     } else {
       return SparseJoinWithSeparatorJS(array, keys, length, convert, separator);
     }
   }
 
   // Fast case for one-element arrays.
   if (length === 1) {
-    return convert(array[0]);
+    var e = array[0];
+    return IS_STRING(e) ? e : convert(e);
   }
 
   // Construct an array for the elements.
   var elements = new InternalArray(length);
 
   // We pull the empty separator check outside the loop for speed!
   if (separator === '') {
     for (var i = 0; i < length; i++) {
-      elements[i] = convert(array[i]);
+      var e = array[i];
+      elements[i] = IS_STRING(e) ? e : convert(e);
     }
     return %StringBuilderConcat(elements, length, '');
   }
   // Non-empty separator case.
-  elements[0] = convert(array[0]);
-  for (var i = 1; i < length; i++) {
-    elements[i] = convert(array[i]);
+  // If the first element is a number then use the heuristic that the
+  // remaining elements are also likely to be numbers.
+  var e = array[0];
+  if (IS_NUMBER(e)) {
+    elements[0] = %_NumberToString(e);
+    for (var i = 1; i < length; i++) {
+      e = array[i];
+      if (IS_NUMBER(e)) {
+        elements[i] = %_NumberToString(e);
+      } else {
+        elements[i] = IS_STRING(e) ? e : convert(e);
+      }
+    }
+  } else {
+    elements[0] = IS_STRING(e) ? e : convert(e);
+    for (var i = 1; i < length; i++) {
+      e = array[i];
+      elements[i] = IS_STRING(e) ? e : convert(e);
+    }
   }
   return %StringBuilderJoin(elements, length, separator);
 }
 
 function Join(array, length, separator, convert) {
   if (length === 0) return '';
 
   var is_array = IS_ARRAY(array);
 
   if (is_array) {
@@ skipping 1579 matching lines @@
 %InstallToContext([
   "array_pop", ArrayPop,
   "array_push", ArrayPush,
   "array_shift", ArrayShift,
   "array_splice", ArraySplice,
   "array_slice", ArraySlice,
   "array_unshift", ArrayUnshift,
 ]);
 
 });