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

tests/lib/platform/environment_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.environment, new isInstanceOf<Map<String, String>>());
+  } else {
+    expect(platform.environment, isNull);
+  }
+}
+