dart:platform | Remove isWindows, isLinux, isAndroid, isMacOS from dart:platform.

runtime/lib/platform_patch.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.
 
 patch int get numberOfProcessors => _platform.numberOfProcessors;
 
 patch String get pathSeparator => _platform.pathSeparator;
 
 patch String get operatingSystem => _platform.operatingSystem;
 
 patch String get localHostname => _platform.localHostname;
 
 patch String get version => _platform.version;
 
 patch Map<String, String> get environment => _platform.environment;
 
 patch String get executable => _platform.executable;
 
 patch Uri get script => _platform.script;
 
 patch List<String> get executableArguments => _platform.executableArguments;
 
 patch String get packageRoot => _platform.packageRoot;
 
-patch bool get isLinux => _platform.operatingSystem == "linux";
-
-patch bool get isMacOS => _platform.operatingSystem == "macos";
-
-patch bool get isWindows => _platform.operatingSystem == "windows";
-
-patch bool get isAndroid => _platform.operatingSystem == "android";
-
 class _Platform {
   int get numberOfProcessors;
   String get pathSeparator;
   String get operatingSystem;
   String get localHostname;
   String get version;
   Map get environment;
   Uri get script;
   String get executable;
   List<String>  get executableArguments;
   String get packageRoot;
 }
 
 // A non-default _Platform, with real values, is stored here by the embedder.
 _Platform _platform = new _DefaultPlatform();
 
 class _DefaultPlatform implements _Platform {
   int get numberOfProcessors {
     return null;
   }
 
   String get pathSeparator {
-    return "/";
+    return null;
   }
 
   String get operatingSystem {
     return null;
   }
 
   String get localHostname {
     return null;
   }
 
@@ skipping 14 matching lines @@
   }
 
   List<String>  get executableArguments {
     return new List<String>(0);
   }
 
   String get packageRoot {
     return null;
   }
 }