Make the Platform.environment return value an unmodifiable map

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 45 matching lines @@
   static bool get isWindows => _operatingSystem == "windows";
 
   /**
    * Returns true if the operating system is Android.
    */
   static bool get isAndroid => _operatingSystem == "android";
 
   /**
    * Get the environment for this process.
    *
+   * The returned environment is a unmodifiable map which content is
+   * retrieved from the operating system on its first use.
+   *
    * 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;
 
   /**
    * Returns the path of the executable used to run the script in this
    * isolate.
@@ skipping 36 matching lines @@
    *
    * If there is no --package-root flag, then the empty string is returned.
    */
   static String get packageRoot => _Platform.packageRoot;
 
   /**
    * Returns the version of the current Dart runtime.
    */
   static String get version => _version;
 }