Reorganize tests for samples so the test is discoverable from the sample

samples/swarm/test/swarm_ui_lib/observable/change_event_tests.dart

 // Copyright (c) 2011, 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.
 
 part of observable_tests;
 
 testChangeEvent() {
   test('constructor', () {
     // create property, list, global and check the proper initialization.
     final target = new AbstractObservable();
@@ skipping 12 matching lines @@
 
     validateRemove(
         new ChangeEvent.list(target, ChangeEvent.REMOVE, 3, null, '12'),
         target, null, 3, '12');
 
     validateGlobal(
         new ChangeEvent.list(target, ChangeEvent.GLOBAL, null, null, null),
         target);
   });
 }