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

src/date.js

Index: src/date.js
diff --git a/src/date.js b/src/date.js
index 5e3f5c1a5ebd896fc9443e00c739058c68e0da3b..3f5c8b5c689c1e16b9fe082ad7eb0093c6862cb2 100644
--- a/src/date.js
+++ b/src/date.js
@@ -47,6 +47,7 @@ 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;
@@ -159,6 +160,7 @@ function DateConstructor(year, month, date, hours, minutes, seconds, ms) {
     } 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;
@@ -745,7 +747,19 @@ function DateToJSON(key) {
 }
 
 
-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;