Use patching for dart:io.

runtime/bin/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";
-
-  static int get numberOfProcessors {
-    return _numberOfProcessors();
-  }
-
-  static String get pathSeparator {
-    return _pathSeparator();
-  }
-
-  static String get operatingSystem {
-    return _operatingSystem();
-  }
-
-  static String get localHostname {
-    var result = _localHostname();
-    if (result is OSError) {
-      throw result;
-    } else {
-      return result;
-    }
-  }
-
-  static Map<String, String> get environment {
-    var env = _environment();
-    if (env is OSError) {
-      throw env;
-    } else {
-      var result = new Map();
-      for (var str in env) {
-        // When running on Windows through cmd.exe there are strange
-        // environment variables that are used to record the current
-        // working directory for each drive and the exit code for the
-        // last command. As an example: '=A:=A:\subdir' records the
-        // current working directory on the 'A' drive.  In order to
-        // 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);
-      }
-      return result;
-    }
-  }
-}