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

runtime/vm/os.h

Index: runtime/vm/os.h
diff --git a/runtime/vm/os.h b/runtime/vm/os.h
index 2c0fbb93cc704e202c85b489be3a7dc94ef469bb..490098384157069746296281526e4ef25edb784c 100644
--- a/runtime/vm/os.h
+++ b/runtime/vm/os.h
@@ -42,6 +42,17 @@ class OS {
   // Returns true if the conversion succeeds, false otherwise.
   static bool MkTime(tm* tm, int64_t* seconds_result);
 
+  // Returns the abbreviated time-zone name for the given instant.
+  // For example "CET" or "CEST".
+  static bool GetTimeZoneName(int64_t seconds_since_epoch,
+                              const char** name_result);
+
+  // Returns the difference in seconds between local time and UTC for the given
+  // instant.
+  // For example 3600 for CET, and 7200 for CEST.
+  static bool GetTimeZoneOffsetInSeconds(int64_t seconds_since_epoch,
+                                         int* offset_result);
+
   // Returns the current time in milliseconds measured
   // from midnight January 1, 1970 UTC.
   static int64_t GetCurrentTimeMillis();