Reapply "Add support for timezone offset and timezone name."

runtime/lib/date.dart

Index: runtime/lib/date.dart
diff --git a/runtime/lib/date.dart b/runtime/lib/date.dart
index 3e33c3aa2b266269f0aae52157da485fc6d9c756..97395aed124284628348d47c4de312c9fddf5805 100644
--- a/runtime/lib/date.dart
+++ b/runtime/lib/date.dart
@@ -130,6 +130,18 @@ class DateImplementation implements Date {
     return new Date.fromEpoch(value, targetTimeZone);
   }
 
+  String get timeZoneName() {
+    if (isUtc()) return "UTC";
+    return _timeZoneName(_equivalentSeconds(_secondsSinceEpoch));
+  }
+
+  Duration get timeZoneOffset() {
+    if (isUtc()) return new Duration(0);
+    int offsetInSeconds =
+        _timeZoneOffsetInSeconds(_equivalentSeconds(_secondsSinceEpoch));
+    return new Duration(seconds: offsetInSeconds);
+  }
+
   int get year() {
     int secondsSinceEpoch = _secondsSinceEpoch;
     // According to V8 some library calls have troubles with negative values.
@@ -401,6 +413,12 @@ class DateImplementation implements Date {
 
   static int _getCurrentMs() native "DateNatives_currentTimeMillis";
 
+  static String _timeZoneName(int secondsSinceEpoch)
+      native "DateNatives_timeZoneName";
+
+  static int _timeZoneOffsetInSeconds(int secondsSinceEpoch)
+      native "DateNatives_timeZoneOffsetInSeconds";
+
   // TODO(floitsch): it would be more efficient if we didn't call the native
   // function for every member, but cached the broken-down date.
   static int _getYear(int secondsSinceEpoch, bool isUtc)