Make the Platform.environment return value an unmodifiable map

runtime/bin/process_patch.dart

 // Copyright (c) 2013, 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.
 
 patch class _WindowsCodePageDecoder {
   /* patch */ static String _decodeBytes(List<int> bytes)
       native "SystemEncodingToString";
 }
 
 
@@ skipping 180 matching lines @@
     }
 
     _environment = [];
     if (environment == null) {
       environment = {};
     }
     if (environment is !Map) {
       throw new ArgumentError("Environment is not a map: $environment");
     }
     if (identical(true, includeParentEnvironment)) {
-      environment = Platform.environment..addAll(environment);
+      environment = new Map.from(Platform.environment)..addAll(environment);
     }
     environment.forEach((key, value) {
       if (key is !String || value is !String) {
         throw new ArgumentError(
             "Environment key or value is not a string: ($key, $value)");
       }
       _environment.add('$key=$value');
     });
 
     // stdin going to process.
@@ skipping 320 matching lines @@
   const _ProcessResult(int this.pid,
                        int this.exitCode,
                        this.stdout,
                        this.stderr);
 
   final int pid;
   final int exitCode;
   final stdout;
   final stderr;
 }