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

tests/lib/platform/operating_system_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 != null) {
     expect(platform.operatingSystem,
            isIn(['linux', 'macos', 'windows', 'android']));
   }
-  expect(platform.isLinux, platform.operatingSystem == 'linux');
-  expect(platform.isMacOS, platform.operatingSystem == 'macos');
-  expect(platform.isWindows, platform.operatingSystem == 'windows');
-  expect(platform.isAndroid, platform.operatingSystem == 'android');
 }