Use AbsolutePathContext in Folder.contains().

pkg/analyzer/lib/file_system/file_system.dart

Index: pkg/analyzer/lib/file_system/file_system.dart
diff --git a/pkg/analyzer/lib/file_system/file_system.dart b/pkg/analyzer/lib/file_system/file_system.dart
index 7d3ed95ad2988a45c3f4869e092b71b112056918..21e8d05112df3b3aed1625c5e140ca2077e13431 100644
--- a/pkg/analyzer/lib/file_system/file_system.dart
+++ b/pkg/analyzer/lib/file_system/file_system.dart
@@ -7,6 +7,7 @@ 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';
 
@@ -134,6 +135,11 @@ abstract class Resource {
  */
 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;