Remove uses of unittest in the HTML tests where possible.

pkg/dev_compiler/test/codegen/lib/html/mouse_event_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 mouse_event_test;
-import 'package:unittest/unittest.dart';
-import 'package:unittest/html_config.dart';
 import 'dart:html';
 
+import 'package:expect/minitest.dart';
+
 main() {
-  useHtmlConfiguration();
-
   test('relatedTarget', () {
     var event = new MouseEvent('mouseout');
     expect(event.relatedTarget, isNull);
 
     event = new MouseEvent('mouseout', relatedTarget: document.body);
     expect(event.relatedTarget, document.body);
   });
 }