Package map source factory support.

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 memory_file_system;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:core' hide Resource;
 
@@ skipping 47 matching lines @@
         throw 'failed to delete resource: $child';
       }
     }
     _pathToResource.remove(path);
     _pathToContent.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);
+
+  @override
   Resource getResource(String path) {
     path = posix.normalize(path);
     Resource resource = _pathToResource[path];
     if (resource == null) {
       resource = new _MemoryFile(this, path);
     }
     return resource;
   }
 
   @override
@@ skipping 388 matching lines @@
   bool operator ==(other) {
     if (runtimeType != other.runtimeType) {
       return false;
     }
     return path == other.path;
   }
 
   @override
   String toString() => path;
 }