Use AbsolutePathContext in Folder.contains().

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:analyzer/src/util/absolute_path.dart';
 import 'package:path/path.dart';
 import 'package:watcher/watcher.dart';
 
 /**
  * [File]s are leaf [Resource]s which contain data.
  */
 abstract class File extends Resource {
   /**
    * Watch for changes to this file
    */
@@ skipping 107 matching lines @@
    */
   bool isOrContains(String path);
 }
 
 /**
  * Instances of the class [ResourceProvider] convert [String] paths into
  * [Resource]s.
  */
 abstract class ResourceProvider {
   /**
+   * Get the absolute path context used by this resource provider.
+   */
+  AbsolutePathContext get absolutePathContext;
+
+  /**
    * Get the path context used by this resource provider.
    */
   Context get pathContext;
 
   /**
    * Return a [File] that corresponds to the given [path].
    *
    * A file may or may not exist at this location.
    */
   File getFile(String path);
@@ skipping 48 matching lines @@
 
   @override
   Uri restoreAbsolute(Source source) =>
       _provider.pathContext.toUri(source.fullName);
 
   /**
    * Return `true` if the given [uri] is a `file` URI.
    */
   static bool _isFileUri(Uri uri) => uri.scheme == _FILE_SCHEME;
 }