Improve [relativize] for Windows.

dart/utils/compiler/build_helper.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.
 
 #import('dart:io');
 #import('dart:uri');
 
 #import('../../lib/compiler/implementation/util/uri_extras.dart');
 #import('../../lib/compiler/implementation/filenames.dart');
 
@@ skipping 61 matching lines @@
         exit(result.exitCode);
       }
     }
     Process.run('/bin/chmod', ['+x', uri.path]).then(onExit);
   }
 }
 
 List<String> buildScript(String name,
                          Uri dartUri, Uri dartVmLocation,
                          String entrypoint, String options) {
+  bool isWindows = (Platform.operatingSystem == 'windows');
   Uri uri = dartUri.resolve(entrypoint);
-  String path = relativize(dartVmLocation, uri);
+  String path = relativize(dartVmLocation, uri, isWindows);
   String pathWin = path.replaceAll("/", "\\");
 
   print('dartUri = $dartUri');
   print('dartVmLocation = $dartVmLocation');
   print('${name}Uri = $uri');
   print('${name}Path = $path');
   print('${name}PathWin = $pathWin');
 
   // Tell the VM to grow the heap more aggressively. This should only
   // be necessary temporarily until the VM is better at detecting how
@@ skipping 28 matching lines @@
 set SCRIPTPATH=%~dp0
 
 REM Does the path have a trailing slash? If so, remove it.
 if %SCRIPTPATH:~-1%==\ set SCRIPTPATH=%SCRIPTPATH:~0,-1%
 
 set arguments=%*
 
 "%SCRIPTPATH%\dart.exe"$options "%SCRIPTPATH%$pathWin" %arguments%
 '''.replaceAll('\n', '\r\n')];
 }