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

tests/lib/platform/local_hostname_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.localHostname, new isInstanceOf<String>());
+    expect(platform.localHostname, isNot(''));
+  } else {
+    expect(platform.localHostname, isNull);
+  }
+}