Fix handling of Windows paths after cleanup.

runtime/bin/builtin.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.
 
 library builtin;
 import 'dart:io';
 import 'dart:async';
 import 'dart:convert';
 // import 'root_library'; happens here from C Code
 
@@ skipping 266 matching lines @@
   _asyncLoadErrorCallback(uri, libraryUri, error);
   if (_numOutstandingLoadRequests == 0) {
     _signalDoneLoading();
     _cleanup();
   }
 }
 
 
 // Asynchronously loads script data through a http or file uri.
 _loadDataAsync(int tag, String uri, String libraryUri) {
-  var filePath = _filePathFromUri(uri);
+  var filePath;
   Uri sourceUri;
   if (tag == null) {
     uri = _resolveScriptUri(uri);
     sourceUri = Uri.parse(uri);
+    filePath = _filePathFromUri(uri);
   } else {
+    filePath = _filePathFromUri(uri);
     sourceUri = Uri.parse(filePath);
   }
   _numOutstandingLoadRequests++;
   _logResolution("_loadDataAsync($uri), "
                  "${_numOutstandingLoadRequests} requests outstanding");
   if (sourceUri.scheme == 'http') {
     _httpGet(sourceUri, libraryUri, (data) {
       _loadScript(tag, uri, libraryUri, data);
     });
   } else {
@@ skipping 45 matching lines @@
   } else if (Platform.isWindows) {
     filename = '$name.dll';
   } else {
     _logResolution(
         'Native extensions not supported on ${Platform.operatingSystem}');
     throw 'Native extensions not supported on ${Platform.operatingSystem}';
   }
 
   return [path, filename, name];
 }