Add support for cross-session storage

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 'package:analyzer/src/generated/engine.dart' show TimestampedData;
@@ skipping 47 matching lines @@
   @override
   Resource getResource(String path) {
     path = posix.normalize(path);
     Resource resource = _pathToResource[path];
     if (resource == null) {
       resource = new _MemoryFile(this, path);
     }
     return resource;
   }
 
+  @override
+  Folder getStateLocation(String pluginId) {
+    return newFolder('/user/home/$pluginId');

[Brian Wilkerson, 2014/10/07 15:55:23]

I'm tempted to use an invalid folder name in the path to avoid collisions, but I
don't know what the effect of that would be.

+  }
+
   void modifyFile(String path, String content) {
     _checkFileAtPath(path);
     _pathToContent[path] = content;
     _pathToTimestamp[path] = nextStamp++;
     _notifyWatchers(path, ChangeType.MODIFY);
   }
 
   /**
    * Create a resource representing a dummy link (that is, a File object which
    * appears in its parent directory, but whose `exists` property is false)
@@ skipping 296 matching lines @@
   bool operator ==(other) {
     if (runtimeType != other.runtimeType) {
       return false;
     }
     return path == other.path;
   }
 
   @override
   String toString() => path;
 }