Remove unmodifiable map wrappers that could instead use UnmodifiableMapView.

sdk/lib/io/platform_impl.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;
 
 class _Platform {
   external static int _numberOfProcessors();
   external static String _pathSeparator();
   external static String _operatingSystem();
@@ skipping 53 matching lines @@
           // handle these correctly we search for a second occurrence of
           // of '=' in the string if the first occurrence is at index 0.
           var equalsIndex = str.indexOf('=');
           if (equalsIndex == 0) {
             equalsIndex = str.indexOf('=', 1);
           }
           assert(equalsIndex != -1);
           result[str.substring(0, equalsIndex)] =
               str.substring(equalsIndex + 1);
         }
-        _environmentCache = new _UnmodifiableMap(result);
+        _environmentCache = new UnmodifiableMapView(result);
       } else {
         _environmentCache = env;
       }
     }
 
     if (_environmentCache is OSError) {
       throw _environmentCache;
     } else {
       return _environmentCache;
     }
@@ skipping 30 matching lines @@
   }
   V remove(String key) => _map.remove(key.toUpperCase());
   void clear() => _map.clear();
   void forEach(void f(String key, V value)) => _map.forEach(f);
   Iterable<String> get keys => _map.keys;
   Iterable<V> get values => _map.values;
   int get length => _map.length;
   bool get isEmpty => _map.isEmpty;
   bool get isNotEmpty => _map.isNotEmpty;
 }