Make the Platform.environment return value an unmodifiable map

tests/standalone/io/process_environment_test.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.
 
 import "dart:io";
 
 import "package:async_helper/async_helper.dart";
 import "package:expect/expect.dart";
 
 import "process_test_util.dart";
@@ skipping 47 matching lines @@
     // Only check one value to not spin up too many processes testing the
     // same things.
     break;
   }
 }
 
 testNoIncludeEnvironment() {
   asyncStart();
   var env = Platform.environment;
   Expect.isTrue(env.containsKey('PATH'));
+  env = new Map.from(env);
   env.remove('PATH');
   runEnvironmentProcess(env, "PATH", false, (output) {
     Expect.isTrue(output.startsWith("null"));
     asyncEnd();
   });
 }
 
 main() {
   testEnvironment();
   testNoIncludeEnvironment();
 }