Fix a bug in pathToFileUri.

pkg/stack_trace/lib/src/utils.dart

 // Copyright (c) 2013, 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 utils;
 
 import 'dart:io';
 import 'dart:uri';
 
 import 'package:pathos/path.dart' as path;
@@ skipping 12 matching lines @@
     // Network paths look like "file://hostname/path/to/file".
     return "\\\\${uri.path.replaceAll("/", "\\")}";
   }
 }
 
 /// Converts a local path string to a `file:` [Uri].
 Uri pathToFileUri(String pathString) {
   pathString = path.absolute(pathString);
   if (Platform.operatingSystem != 'windows') {
     return Uri.parse('file://$pathString');
-  } else if (path.rootPrefix(path).startsWith('\\\\')) {
+  } else if (path.rootPrefix(pathString).startsWith('\\\\')) {
     // Network paths become "file://hostname/path/to/file".
     return Uri.parse('file:${pathString.replaceAll("\\", "/")}');
   } else {
     // Drive-letter paths become "file:///C:/path/to/file".
     return Uri.parse('file:///${pathString.replaceAll("\\", "/")}');
   }
 }