Deprecate dart:io Platform, replaced by dart:platform.

tests/lib/platform/path_separator_test.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.
 
 import "dart:platform" as platform;
 
 import "package:unittest/unittest.dart";
 
 main() {
   if (platform.operatingSystem == 'windows') {
     expect(platform.pathSeparator, '\\');
   } else {
-    expect(platform.pathSeparator, '/');
+    expect(platform.pathSeparator, isIn(['/', null]));
   }
 }