add custom_element package

pkg/observe/test/observe_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:async';
 import 'package:logging/logging.dart';
 import 'package:observe/observe.dart';
 import 'package:observe/src/dirty_check.dart' as dirty_check;
 import 'package:unittest/unittest.dart';
 import 'observe_test_utils.dart';
 
+// Note: this ensures we run the dartanalyzer on the @observe package.
+// @static-clean
+
 const _VALUE = const Symbol('value');
 
 main() {
   // Note: to test the basic Observable system, we use ObservableBox due to its
   // simplicity. We also test a variant that is based on dirty-checking.
 
   observeTest('no observers at the start', () {
     expect(dirty_check.allObservablesCount, 0);
   });
 
@@ skipping 219 matching lines @@
 _changedValue(len) => new List.filled(len, new PropertyChangeRecord(_VALUE));
 
 // A test model based on dirty checking.
 class WatcherModel<T> extends ObservableBase {
   @observable T value;
 
   WatcherModel([T initialValue]) : value = initialValue;
 
   String toString() => '#<$runtimeType value: $value>';
 }