Remove uses of unittest in the HTML tests where possible.

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

 // Copyright (c) 2016, 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.
 
 import 'package:expect/expect.dart';
 
 import 'dart:html';
 
 main() {
   // Regression test for: https://github.com/dart-lang/dev_compiler/issues/508.
   // "dart:html" defines some private members on native DOM types and we need
   // to ensure those can be accessed correctly.
   //
   // The createFragment() method sets `_innerHtml` on the element, so we use it
   // as a test case.
-  Expect.equals("[object DocumentFragment]",
+  Expect.equals("Instance of 'DocumentFragment'",
       new BRElement().createFragment("Hi").toString());
 }