Use patching for dart:io.

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.
 
 class _Platform {
-  static int _numberOfProcessors() native "Platform_NumberOfProcessors";
-  static String _pathSeparator() native "Platform_PathSeparator";
-  static String _operatingSystem() native "Platform_OperatingSystem";
-  static _localHostname() native "Platform_LocalHostname";
-  static _environment() native "Platform_Environment";
+  external static int _numberOfProcessors();
+  external static String _pathSeparator();
+  external static String _operatingSystem();
+  external static _localHostname();
+  external static _environment();
 
   static int get numberOfProcessors {
     return _numberOfProcessors();
   }
 
   static String get pathSeparator {
     return _pathSeparator();
   }
 
   static String get operatingSystem {
@@ skipping 27 matching lines @@
         if (equalsIndex == 0) {
           equalsIndex = str.indexOf('=', 1);
         }
         assert(equalsIndex != -1);
         result[str.substring(0, equalsIndex)] = str.substring(equalsIndex + 1);
       }
       return result;
     }
   }
 }