Use _provider.pathContext to convert URIs to files.

pkg/analyzer/lib/file_system/file_system.dart

 // Copyright (c) 2014, 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 file_system;
 
 import 'dart:async';
 
 import 'package:analyzer/src/generated/source.dart';
 import 'package:path/path.dart';
@@ skipping 112 matching lines @@
 
   final ResourceProvider _provider;
 
   ResourceUriResolver(this._provider);
 
   @override
   Source resolveAbsolute(Uri uri) {
     if (!_isFileUri(uri)) {
       return null;
     }
-    Resource resource = _provider.getResource(fromUri(uri));
+    Resource resource = _provider.getResource(_provider.pathContext.fromUri(uri)
+        );
     if (resource is File) {
       return resource.createSource(uri);
     }
     return null;
   }
 
   /**
    * Return `true` if the given URI is a `file` URI.
    *
    * @param uri the URI being tested
    * @return `true` if the given URI is a `file` URI
    */
   static bool _isFileUri(Uri uri) => uri.scheme == _FILE_SCHEME;
 }