Make the Platform.environment return value an unmodifiable map

tools/testing/dart/test_runner.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.
 
 /**
  * Classes and methods for executing tests.
  *
  * This module includes:
  * - Managing parallel execution of tests, including timeout checks.
  * - Evaluating the output of each test as pass/fail/crash/timeout.
@@ skipping 1434 matching lines @@
         new DateTime.now().difference(startTime),
         compilationSkipped);
     return commandOutput;
   }
 
   Future _drainStream(Stream<List<int>> source, List<int> destination) {
     return source.listen(destination.addAll).asFuture();
   }
 
   Map<String, String> _createProcessEnvironment() {
-    var environment = io.Platform.environment;
+    var environment = new Map.from(io.Platform.environment);
 
     if (command.environmentOverrides != null) {
       for (var key in command.environmentOverrides.keys) {
         environment[key] = command.environmentOverrides[key];
       }
     }
     for (var excludedEnvironmentVariable in EXCLUDED_ENVIRONMENT_VARIABLES) {
       environment.remove(excludedEnvironmentVariable);
     }
 
@@ skipping 1003 matching lines @@
     }
   }
 
   void eventAllTestsDone() {
     for (var listener in _eventListener) {
       listener.allDone();
     }
     _allDone();
   }
 }