Fixes for context_manager_test on Windows.

pkg/analyzer/lib/source/config.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.
 
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/source/analysis_options_provider.dart';
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/task/model.dart';
 import 'package:analyzer/task/model.dart';
 import 'package:package_config/packages.dart';
@@ skipping 73 matching lines @@
           if (config is YamlMap) {
             AnalysisConfigurationDescriptor descriptor =
                 new AnalysisConfigurationDescriptor.fromAnalyzerOptions(config);
 
             if (descriptor.isValid) {
               //Create a path, given descriptor and packagemap
               Uri uri = packages.asMap()[descriptor.package];
               Uri pragma = new Uri.file('config/${descriptor.pragma}.yaml',
                   windows: false);
               Uri optionsUri = uri.resolveUri(pragma);
-              File file = resourceProvider.getFile(optionsUri.toFilePath());
+              String path = resourceProvider.pathContext.fromUri(optionsUri);
+              File file = resourceProvider.getFile(path);
               if (file.exists) {
                 return optionsProvider.getOptionsFromFile(file);
               }
             }
           }
         }
       } catch (_) {
         // Skip exceptional configurations.
       }
     }
     return null;
   }
 }