Fix path handling in tools/version.dart so that it does not matter where we call this from.

tools/testing/dart/version.dart

 // Copyright (c) 2012, 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.
 
 /**
  * This file contains functionality for getting dart version numbers using
  * our standard version construction method. Systems that does not include this
  * file should emulate the structure for revision numbers that we have here.
  *
  * The version number of a dart build is constructed as follows:
@@ skipping 17 matching lines @@
  */
 class Version {
   String _versionFileName;
   String USERNAME;
   int REVISION;
   int MAJOR;
   int MINOR;
   int BUILD;
   int PATCH;
 
-  Version(String this._versionFileName);
+  Version(Path versionFile) {
+    _versionFileName = versionFile.toNativePath();
+  }
 
   /**
    * Get the version number for this specific build using the version info
    * from the VERSION file in the root directory and the revision info
    * from the source control system of the current checkout.
    */
   Future<String> getVersion() {
     File f = new File(_versionFileName);
     Completer c = new Completer();
     f.exists().then((existed) {
@@ skipping 119 matching lines @@
   static RepositoryType guessType() {
     if (new Directory(".svn").existsSync()) return RepositoryType.SVN;
     if (new Directory(".git").existsSync()) return RepositoryType.GIT;
     return RepositoryType.UNKNOWN;
   }
 
   String toString() => name;
 
   final String name;
 }