Make toLocaleString on arrays always call toLocaleString on its elements.

test/mjsunit/array-tostring.js

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 107 matching lines @@
 // Testing Array.prototype.toLocaleString
 
 // Ensure that it never uses Array.prototype.toString for anything.
 Array.prototype.toString = function() { assertUnreachable(); };
 
 // Default case.
 var la1 = [1, [2, 3], 4];
 assertEquals("1,2,3,4", la1.toLocaleString());
 
 // Used on a string (which looks like an array of characters).
-String.prototype.toLocaleString = Array.prototype.toLocaleString;
+String.prototype.toLocaleString = function() {
+  return (this.length == 1) ? this : Array.prototype.toLocaleString.call(this);
+}
 assertEquals("1,2,3,4", "1234".toLocaleString());
 
 // If toLocaleString of element is not callable, throw a TypeError.
 var la2 = [1, {toLocaleString: "not callable"}, 3];
 assertThrows(function() { la2.toLocaleString(); }, TypeError);
 
 // If toLocaleString of element is callable, call it.
 var la3 = [1, {toLocaleString: function() { return "XX";}}, 3];
 assertEquals("1,XX,3", la3.toLocaleString());
 
@@ skipping 11 matching lines @@
     }});
 for (var i = 0; i < 3; i++) {
   Object.defineProperty(Number.prototype, i, {
       get: function() {
         assertEquals(expectedThis, this);
         return +this;
       }});
 }
 Number.prototype.arrayToLocaleString = Array.prototype.toLocaleString;
 assertEquals("42,42,42", (42).arrayToLocaleString());
+
+
+(function TestToLocaleStringCalls() {
+  let log = [];
+  let pushArgs = (label) => (...args) => log.push(label, args);
+
+  let NumberToLocaleString = Number.prototype.toLocaleString;
+  let StringToLocaleString = String.prototype.toLocaleString;
+  let ObjectToLocaleString = Object.prototype.toLocaleString;
+  Number.prototype.toLocaleString = pushArgs("Number");
+  String.prototype.toLocaleString = pushArgs("String");
+  Object.prototype.toLocaleString = pushArgs("Object");
+
+  [42, "foo", {}].toLocaleString();
+  assertEquals(["Number", [], "String", [], "Object", []], log);
+
+  Number.prototype.toLocaleString = NumberToLocaleString;
+  String.prototype.toLocaleString = StringToLocaleString;
+  Object.prototype.toLocaleString = ObjectToLocaleString;
+})();