Remove uses of unittest in the HTML tests where possible.

pkg/dev_compiler/test/codegen/lib/html/custom/document_register_type_extensions_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 document_register_type_extensions_test;
-import 'package:unittest/unittest.dart';
-import 'package:unittest/html_individual_config.dart';
 import 'dart:html';
+
+import 'package:expect/minitest.dart';
+
 import '../utils.dart';
 
 class Foo extends HtmlElement {
   static const tag = 'x-foo';
   static final List outerHtmlStrings = [
     '<x-foo></x-foo>',
     '<?XML:NAMESPACE PREFIX = PUBLIC NS = "URN:COMPONENT" /><x-foo></x-foo>'];
   factory Foo() => new Element.tag(tag);
   Foo.created() : super.created();
 }
@@ skipping 289 matching lines @@
 
   group('functional', () {
     setUp(registerTypes);
 
     test('canvas', () {
       var canvas = new MyCanvas();
       canvas.fillAsRed();
     });
   });
 }