Remove uses of unittest in the HTML tests where possible.

pkg/dev_compiler/test/codegen/lib/html/mouse_event_test.dart

Index: pkg/dev_compiler/test/codegen/lib/html/mouse_event_test.dart
diff --git a/pkg/dev_compiler/test/codegen/lib/html/mouse_event_test.dart b/pkg/dev_compiler/test/codegen/lib/html/mouse_event_test.dart
index 38792c27257a0093da2c6c9f518ccfba2a4e401f..7d7b0d9c33b1f951e3e2dab0b6ce133fc27425c3 100644
--- a/pkg/dev_compiler/test/codegen/lib/html/mouse_event_test.dart
+++ b/pkg/dev_compiler/test/codegen/lib/html/mouse_event_test.dart
@@ -2,14 +2,11 @@
 // 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';
 
-main() {
-  useHtmlConfiguration();
+import 'package:expect/minitest.dart';
 
+main() {
   test('relatedTarget', () {
     var event = new MouseEvent('mouseout');
     expect(event.relatedTarget, isNull);