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

samples/tests/samples/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();
-
-    validateUpdate(
-        new ChangeEvent.property(target, 'pK', 33, '12'),
-        target, 'pK', null, 33, '12');
-
-    validateUpdate(
-        new ChangeEvent.list(target, ChangeEvent.UPDATE, 3, 33, '12'),
-        target, null, 3, 33, '12');
-
-    validateInsert(
-        new ChangeEvent.list(target, ChangeEvent.INSERT, 3, 33, null),
-        target, null, 3, 33);
-
-    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);
-  });
-}