Fix Windows time zone name extraction. Update API docs

sdk/lib/core/date_time.dart

Index: sdk/lib/core/date_time.dart
diff --git a/sdk/lib/core/date_time.dart b/sdk/lib/core/date_time.dart
index 35e93e004596b49cb7627144bd7cbd03208bfdda..b5e9c3ce26f6376904a5e306531759be0f235f49 100644
--- a/sdk/lib/core/date_time.dart
+++ b/sdk/lib/core/date_time.dart
@@ -664,8 +664,10 @@ class DateTime implements Comparable<DateTime> {
   external int get microsecondsSinceEpoch;
 
   /**
-   * The abbreviated time zone name&mdash;for example,
-   * [:"CET":] or [:"CEST":].
+   * The time zone name provided by the platform.
+   *
+   * On Unix-like systems this will probably be an abbreviation. On Windows
+   * this will probably be the full-name, e.g. "Pacific Standard Time".
    */
   external String get timeZoneName;