Fix type warning in platform_impl.dart.

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 65 matching lines @@
   V operator [](String key) => _map[key.toUpperCase()];
   void operator []=(String key, V value) {
     _map[key.toUpperCase()] = value;
   }
   V putIfAbsent(String key, V ifAbsent()) {
     _map.putIfAbsent(key.toUpperCase(), ifAbsent);
   }
   V remove(String key) => _map.remove(key.toUpperCase());
   void clear() => _map.clear();
   void forEach(void f(String key, V value)) => _map.forEach(f);
-  Collection<String> get keys => _map.keys;
-  Collection<V> get values => _map.values;
+  Iterable<String> get keys => _map.keys;
+  Iterable<V> get values => _map.values;
   int get length => _map.length;
   bool get isEmpty => _map.isEmpty;
 
   Map<String, V> _map;
 }