Use a per-isolate cache for the date object JS bits

src/date.js

 // Copyright 2006-2008 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 29 matching lines @@
   throw new $TypeError('this is not a Date object.');
 }
 
 
 var timezone_cache_time = NAN;
 var timezone_cache_timezone_offset = NAN;
 var timezone_cache_timezone;
 
 function LocalTimezone(t, timezone_offset) {
   if (NUMBER_IS_NAN(t)) return "";
+  CheckDateCacheCurrent();
   if (t == timezone_cache_time &&
       timezone_offset == timezone_cache_timezone_offset) {
     return timezone_cache_timezone;
   }
   var timezone = %DateLocalTimezone(t);
   timezone_cache_time = t;
   timezone_cache_timezone = timezone;
   timezone_cache_timezone_offset = timezone_offset;
   return timezone;
 }
@@ skipping 92 matching lines @@
   var value;
   if (argc == 0) {
     value = %DateCurrentTime();
     SET_UTC_DATE_VALUE(this, value);
   } else if (argc == 1) {
     if (IS_NUMBER(year)) {
       value = year;
     } else if (IS_STRING(year)) {
       // Probe the Date cache. If we already have a time value for the
       // given time, we re-use that instead of parsing the string again.
+      CheckDateCacheCurrent();
       var cache = Date_cache;
       if (cache.string === year) {
         value = cache.time;
       } else {
         value = DateParse(year);
         if (!NUMBER_IS_NAN(value)) {
           cache.time = value;
           cache.string = year;
         }
       }
@@ skipping 566 matching lines @@
 function DateToJSON(key) {
   var o = ToObject(this);
   var tv = DefaultNumber(o);
   if (IS_NUMBER(tv) && !NUMBER_IS_FINITE(tv)) {
     return null;
   }
   return o.toISOString();
 }
 
 
-function ResetDateCache() {
+var date_cache_version_holder;
+var date_cache_version = NAN;
+
+
+function CheckDateCacheCurrent() {
+  if (!date_cache_version_holder) {
+    date_cache_version_holder = %DateCacheVersion();
+  }
+  if (date_cache_version_holder[0] == date_cache_version) {
+    return;
+  }
+  date_cache_version = date_cache_version_holder[0];
+
   // Reset the timezone cache:
   timezone_cache_time = NAN;
   timezone_cache_timezone = undefined;
 
   // Reset the date cache:
   cache = Date_cache;
   cache.time = NAN;
   cache.string = null;
 }
 
@@ skipping 62 matching lines @@
     "toGMTString", DateToGMTString,
     "toUTCString", DateToUTCString,
     "getYear", DateGetYear,
     "setYear", DateSetYear,
     "toISOString", DateToISOString,
     "toJSON", DateToJSON
   ));
 }
 
 SetUpDate();