Use %_IsMinusZero where applicable to replace hackery.

src/i18n.js

 // Copyright 2013 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 1284 matching lines @@
 %SetNativeFlag(Intl.NumberFormat.supportedLocalesOf);
 
 
 /**
  * Returns a String value representing the result of calling ToNumber(value)
  * according to the effective locale and the formatting options of this
  * NumberFormat.
  */
 function formatNumber(formatter, value) {
   // Spec treats -0 and +0 as 0.
-  var number = $Number(value);
-  if (number === -0) {
-    number = 0;
-  }
+  var number = $Number(value) + 0;
 
   return %InternalNumberFormat(formatter.formatter, number);
 }
 
 
 /**
  * Returns a Number that represents string value that was passed in.
  */
 function parseNumber(formatter, value) {
   return %InternalNumberParse(formatter.formatter, $String(value));
@@ skipping 788 matching lines @@
   writable: true,
   configurable: true,
   enumerable: false
 });
 %FunctionSetName($Date.prototype.toLocaleTimeString, 'toLocaleTimeString');
 %FunctionRemovePrototype($Date.prototype.toLocaleTimeString);
 %SetNativeFlag($Date.prototype.toLocaleTimeString);
 
 return Intl;
 }())});