[beta][vm] Fix DateTime.timeZoneName on Windows

It needs to look at the given moment to decide whether to use
summer time zone name or standard time zone name.

Previously it was looking at the current time to make this decision
which produced incorrect result: e.g. given
`DateTime.parse(2012-01-02T13:45:23)` its `timeZoneName` should be
returning standard name corresponding to the current time zone even
if we are currently running in summer time (e.g. it should
return PST if machine it is running on is in PDT).

This is revealed by a test which started to fail on Windows
because our Windows bots entered PDT.

Fixes #55159

TEST=corelib/date_time7_test.dart

CoreLibraryReviewExempt: No core library changes.
Cherry-pick: https://dart-review.googlesource.com/c/sdk/+/356681
Cherry-pick-request: #55240
Change-Id: I11fb963ec90055db168c7f34bdf7c58229085bd1
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/358440
Reviewed-by: Siva Annamalai <asiva@google.com>
Commit-Queue: Kevin Chisholm <kevinjchisholm@google.com>

runtime/vm/os_win.cc

@@ -112,13 +112,11 @@ const char* OS::GetTimeZoneName(int64_t seconds_since_epoch) {
   }
 
   // Figure out whether we're in standard or daylight.
-  bool daylight_savings = (status == TIME_ZONE_ID_DAYLIGHT);
-  if (status == TIME_ZONE_ID_UNKNOWN) {
-    tm local_time;
-    if (LocalTime(seconds_since_epoch, &local_time)) {
-      daylight_savings = (local_time.tm_isdst == 1);
-    }
+  tm local_time;
+  if (!LocalTime(seconds_since_epoch, &local_time)) {
+    return "";
   }
+  const bool daylight_savings = (local_time.tm_isdst == 1);
 
   // Convert the wchar string to a null-terminated utf8 string.
   wchar_t* wchar_name = daylight_savings ? zone_information.DaylightName

tests/corelib/date_time7_test.dart

@@ -34,6 +34,8 @@ testLocal() {
         Expect.equals(-4, offset.inHours);
       case "PDT" || "Pacific Daylight Time":
         Expect.equals(-7, offset.inHours);
+      case "PST" || "Pacific Standard Time":
+        Expect.equals(-8, offset.inHours);
       case "CST" || "Central Standard Time":
         Expect.equals(-6, offset.inHours);
       case "CDT" || "Central Daylight Time":