Support for observable models, fixes #259

test/data/input/recursive_test_data.dart

 // Copyright (c) 2012, 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.
 
 library recursive_test_data;
 
 import 'package:unittest/unittest.dart';
+import 'package:web_ui/observe.dart';
 
-int len = 0;
+@observable int len = 0;
 
 String check(pos) {
   expect(pos, lessThanOrEqualTo(len));
   return '$pos';
 }