Make Platform.environment case-insensitive on Windows.

sdk/lib/io/platform.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;
 
 /**
  * The [Platform] class exposes details of the machine and operating
  * system.
  */
@@ skipping 15 matching lines @@
    */
   static String get operatingSystem => _Platform.operatingSystem;
 
   /**
    * Get the local hostname for the system.
    */
   static String get localHostname => _Platform.localHostname;
 
   /**
    * Get the environment for this process.
+   *
+   * Environment variables on Windows are case-insensitive. The map
+   * returned on Windows is therefore case-insensitive and will convert
+   * all keys to upper case. On other platforms the returned map is
+   * a standard case-sensitive map.
    */
   static Map<String, String> get environment => _Platform.environment;
 }