Fix BazelFileUriResolver tests

pkg/analyzer/lib/src/generated/bazel.dart

 // Copyright (c) 2016, 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 analyzer.src.generated.bazel;
 
 import 'dart:core';
 
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/src/generated/source.dart';
@@ skipping 39 matching lines @@
     for (Folder buildDir in _buildDirectories) {
       File file = buildDir.getChildAssumingFile(relativeFromWorkspaceDir);
       if (file.exists) {
         return file.createSource(actualUri ?? uri);
       }
     }
     return null;
   }
 
   String _getPathFromWorkspaceDir(Uri uri) {
-    String uriPath = uri.path;
-    String workspacePath = _workspaceDir.path;
-
-    if (uriPath.startsWith(workspacePath) &&
-        workspacePath.length < uriPath.length) {
-      return uriPath.substring(workspacePath.length + 1);
+    try {
+      return provider.pathContext.relative(provider.pathContext.fromUri(uri),
+          from: _workspaceDir.path);
+    } on Exception {
+      return '';
     }
-    return '';
   }
 }