Use AbsolutePathContext in Folder.contains().

pkg/analyzer/lib/file_system/memory_file_system.dart

Index: pkg/analyzer/lib/file_system/memory_file_system.dart
diff --git a/pkg/analyzer/lib/file_system/memory_file_system.dart b/pkg/analyzer/lib/file_system/memory_file_system.dart
index 058f9eb9ef94b35bb562e826e1e1f0d514206ad3..c27639772c233acca2b02427e24c11a2db450f85 100644
--- a/pkg/analyzer/lib/file_system/memory_file_system.dart
+++ b/pkg/analyzer/lib/file_system/memory_file_system.dart
@@ -10,6 +10,7 @@ import 'dart:core' hide Resource;
 
 import 'package:analyzer/src/generated/engine.dart' show TimestampedData;
 import 'package:analyzer/src/generated/source_io.dart';
+import 'package:analyzer/src/util/absolute_path.dart';
 import 'package:path/path.dart';
 import 'package:watcher/watcher.dart';
 
@@ -28,6 +29,9 @@ class MemoryResourceProvider implements ResourceProvider {
       new HashMap<String, List<StreamController<WatchEvent>>>();
   int nextStamp = 0;
 
+  final AbsolutePathContext absolutePathContext =
+      new AbsolutePathContext(posix.separator);
+
   @override
   Context get pathContext => posix;
 
@@ -44,7 +48,7 @@ class MemoryResourceProvider implements ResourceProvider {
 
   /**
    * Delete the folder with the given path
-   * and recurively delete nested files and folders.
+   * and recursively delete nested files and folders.
    */
   void deleteFolder(String path) {
     _checkFolderAtPath(path);