Implementing custom element's attributeChanged callback in Dartium

tests/html/custom/attribute_changed_callback_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.
 
 library attribute_changed_callback_test;
 
 import 'dart:html';
 import 'dart:js' as js;
 import 'package:unittest/html_individual_config.dart';
 import 'package:unittest/unittest.dart';
@@ skipping 74 matching lines @@
       b.attributes['data-v'] = 'x';
       expect(B.invocations, []);
 
       b.attributes.remove('data-v');
       expect(B.invocations, ['data-v: x => null']);
     });
   });
 
   group('unsupported_on_polyfill', () {
     test('add, change ID', () {
+      B.invocations = [];
+
       var b = new B();
       b.id = 'x';
       expect(B.invocations, ['created', 'id: null => x']);
 
       B.invocations = [];
       b.attributes.remove('id');
       expect(B.invocations, ['id: x => null']);
     });
 
     test('add, change classes', () {
       var b = new B();
 
       B.invocations = [];
       b.classes.toggle('u');
       expect(B.invocations, ['class: null => u']);
     });
   });
 }