Get rid of use of dart:html in unittestlib.

client/testing/unittest/unittest.dart

 // Copyright (c) 2011, 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.
 
-// Version of unittest library that uses the html library versions of 'window'
-// and 'document'.
-
-// TODO: When it is possible to use both dom and html in the same program
-// (i.e. using namespaces to distinguish the two global window() getters),
-// remove one of the versions.
-
+/**
+ * A simple unit test library.
+ */
 #library("unittest");
 
-#import("dart:html");
+#import("dart:dom");
 
 #source("unittestsuite.dart");