[dart:io] Adds Platform.operatingSystemVersion

sdk/lib/io/platform_impl.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of dart.io;
 
 class _Platform {
   external static int _numberOfProcessors();
   external static String _pathSeparator();
   external static String _operatingSystem();
+  external static _operatingSystemVersion();
   external static _localHostname();
   external static _executable();
   external static _resolvedExecutable();
 
   /**
    * Retrieve the entries of the process environment.
    *
    * The result is an [Iterable] of strings, where each string represents
    * an environment entry.
    *
@@ skipping 33 matching lines @@
 
   // Cache the OS environment. This can be an OSError instance if
   // retrieving the environment failed.
   static var /*OSError|Map<String,String>*/ _environmentCache;
 
   static int get numberOfProcessors => _numberOfProcessors();
   static String get pathSeparator => _pathSeparator();
   static String get operatingSystem => _operatingSystem();
   static Uri get script => _script();
 
+  static String _cachedOSVersion;
+  static String get operatingSystemVersion {
+    if (_cachedOSVersion == null) {
+      var result = _operatingSystemVersion();
+      if (result is OSError) {
+        throw result;
+      }
+      _cachedOSVersion = result;
+    }
+    return _cachedOSVersion;
+  }
+
   static String get localHostname {
     var result = _localHostname();
     if (result is OSError) {
       throw result;
-    } else {
-      return result;
     }
+    return result;
   }
 
   static List<String> get executableArguments => _executableArguments();
 
   static Map<String, String> get environment {
     if (_environmentCache == null) {
       var env = _environment();
       if (env is! OSError) {
         var isWindows = operatingSystem == 'windows';
         var result = isWindows
@@ skipping 60 matching lines @@
     _map.forEach(f);
   }
 
   Iterable<String> get keys => _map.keys;
   Iterable<V> get values => _map.values;
   int get length => _map.length;
   bool get isEmpty => _map.isEmpty;
   bool get isNotEmpty => _map.isNotEmpty;
   String toString() => _map.toString();
 }