Don't use relativeTo() since it barfs on some arguments.

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.
 
 part of test_suite;
 
 String getHtmlContents(String title,
                        Path controllerScript,
                        Path dartJsScript,
                        String scriptType,
@@ skipping 40 matching lines @@
 String wrapDartTestInLibrary(Path test) =>
 """
 library libraryWrapper;
 part '$test';
 """;
 
 String dartTestWrapper(Path dartHome, Path library) {
   // Tests inside "pkg" import unittest using "package:". All others use a
   // relative path. The imports need to agree, so use a matching form here.
   var unitTest = dartHome.append("pkg/unittest/lib").toString();
-  if (library.relativeTo(dartHome).segments().contains("pkg")) {
+
+  // TODO(rnystrom): Looking in the entire path here is wrong. It should only
+  // consider the relative path within dartHome. Unfortunately,
+  // Path.relativeTo() does not handle cases where library is already a relative
+  // path, and Path.isAbsolute does not work on Windows.
+  if (library.segments().contains("pkg")) {
     unitTest = 'package:unittest';
   }
 
   return """
 library test;
 
 import '$unitTest/unittest.dart' as unittest;
 import '$unitTest/html_config.dart' as config;
 import '${library}' as Test;
 
 main() {
   config.useHtmlConfiguration();
   unittest.group('', Test.main);
 }
 """;
 }