Add fields to platform library, implement them on runtime dart.

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');
+}