Add support for cross-session storage

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 3751c1c171063ab4a91311e92f42f9fc07213f8e..93fe125a901eaa7ea6fce317ed07f0a947b95d4c 100644
--- a/pkg/analyzer/lib/file_system/memory_file_system.dart
+++ b/pkg/analyzer/lib/file_system/memory_file_system.dart
@@ -65,6 +65,11 @@ class MemoryResourceProvider implements ResourceProvider {
     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;