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.

[floitsch, 2016/08/22 16:20:25]

No need to explicitly use the "provided by the platform" in the initial header
line.
Don't use future-tense in dartdocs, unless it happens after the call has
returned.
Talk about the browser. In the browser, even on windows, it's an abbreviation.

Maybe:

The time-zone name.

The actual value is provided by the operating system and may be an abbreviation
or a full name.

In the browser, or on Unix-like systems commonly returns abbreviations, such as
"CET", "CEST". On Windows returns the full name, for example "Pacific Standard
Time".

+   *
+   * 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;