Remove uses of unittest in the HTML tests where possible.

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

 // Copyright (c) 2012, 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 selectelement_test;
-import 'package:unittest/unittest.dart';
-import 'package:unittest/html_config.dart';
 import 'dart:html';
 
+import 'package:expect/minitest.dart';
+
 main() {
-  useHtmlConfiguration();
-
   test('selectedOptions', () {
     var element = new SelectElement();
     element.multiple = false;
     var options = [
       new OptionElement(),
       new DivElement(),
       new OptionElement(data: 'data', value: 'two', selected: true),
       new DivElement(),
       new OptionElement(data: 'data', value: 'two', selected: true),
       new OptionElement(),
@@ skipping 34 matching lines @@
     expect(element.options.last, equals(options[3]));
   });
 
   test('optgroup', () {
     var element = new Element.html(
       '<select>'
         '<option>1</option>'
         '<optgroup>'
           '<option>2</option>'
         '</optgroup>'
-      '</select>');
+      '</select>') as SelectElement;
 
     expect(element.options.length, 2);
     element.selectedIndex = 1;
 
     var optGroup = element.children[1];
     expect(optGroup is OptGroupElement, isTrue);
-    expect(optGroup.children.single.selected, isTrue);
+    expect((optGroup.children.single as OptionElement).selected, isTrue);
     expect(element.selectedOptions, optGroup.children);
   });
 }