MemoryResourceProvider.getFolder() should not create the folder.

pkg/analyzer/lib/file_system/memory_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 analyzer.file_system.memory_file_system;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:convert';
 import 'dart:core';
@@ skipping 77 matching lines @@
     _pathToResource.remove(path);
     _pathToBytes.remove(path);
     _pathToTimestamp.remove(path);
     _notifyWatchers(path, ChangeType.REMOVE);
   }
 
   @override
   File getFile(String path) => new _MemoryFile(this, path);
 
   @override
-  Folder getFolder(String path) => newFolder(path);
+  Folder getFolder(String path) {
+    path = pathContext.normalize(path);
+    if (!pathContext.isAbsolute(path)) {
+      throw new ArgumentError("Path must be absolute : $path");
+    }
+    return new _MemoryFolder(this, path);
+  }
 
   @override
   Future<List<int>> getModificationTimes(List<Source> sources) async {
     return sources.map((source) {
       String path = source.fullName;
       return _pathToTimestamp[path] ?? -1;
     }).toList();
   }
 
   @override
@@ skipping 426 matching lines @@
 
   @override
   get hashCode => path.hashCode;
 
   @override
   Folder get parent {
     String parentPath = _provider.pathContext.dirname(path);
     if (parentPath == path) {
       return null;
     }
-    return _provider.getResource(parentPath);
+    return _provider.getFolder(parentPath);
   }
 
   @override
   String get shortName => _provider.pathContext.basename(path);
 
   @override
   bool operator ==(other) {
     if (runtimeType != other.runtimeType) {
       return false;
     }
     return path == other.path;
   }
 
   @override
   String toString() => path;
 
   @override
   Uri toUri() => _provider.pathContext.toUri(path);
 }