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

tests/lib/platform/package_root_test.dart

Index: tests/lib/platform/package_root_test.dart
diff --git a/tests/lib/platform/package_root_test.dart b/tests/lib/platform/package_root_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..7d5de6a1964259977cf92eab314a0083b0ab8bf8
--- /dev/null
+++ b/tests/lib/platform/package_root_test.dart
@@ -0,0 +1,16 @@
+// 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.packageRoot is String, true);
+    expect(platform.packageRoot, isNot(''));
+  } else {
+    expect(platform.packageRoot, isNull);
+  }
+}