Update unittest to new package layout.

tools/testing/dart/browser_test.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.
 
 String getHtmlContents(String title,
                        String controllerScript,
                        String dartJsScript,
                        String scriptType,
                        String sourceScript) =>
 """
@@ skipping 37 matching lines @@
 String wrapDartTestInLibrary(Path test) =>
 """
 #library('libraryWrapper');
 #source('$test');
 """;
 
 String dartTestWrapper(Path dartHome, Path library) =>
 """
 #library('test');
 
-#import('${dartHome.append('pkg/unittest/unittest.dart')}', prefix: 'unittest');
-#import('${dartHome.append('pkg/unittest/html_config.dart')}',
+#import('${dartHome.append('pkg/unittest/lib/unittest.dart')}', prefix: 'unittest');
+#import('${dartHome.append('pkg/unittest/lib/html_config.dart')}',
         prefix: 'config');
 
 #import('${library}', prefix: "Test");
 
 main() {
   config.useHtmlConfiguration();
   unittest.group('', Test.main);
 }
 """;