Report async errors when they happend, in directory listing.

sdk/lib/io/directory_impl.dart

Index: sdk/lib/io/directory_impl.dart
diff --git a/sdk/lib/io/directory_impl.dart b/sdk/lib/io/directory_impl.dart
index 39f8b2ca16377d7edb2dff45fa6322f6240736fa..f47ef4f90a7a1c5ff441597ba5b783cb9a81383a 100644
--- a/sdk/lib/io/directory_impl.dart
+++ b/sdk/lib/io/directory_impl.dart
@@ -282,16 +282,16 @@ class _Directory implements Directory {
 }
 
 class _AsyncDirectoryLister {
-  const int LIST_FILE = 0;
-  const int LIST_DIRECTORY = 1;
-  const int LIST_LINK = 2;
-  const int LIST_ERROR = 3;
-  const int LIST_DONE = 4;
-
-  const int RESPONSE_TYPE = 0;
-  const int RESPONSE_PATH = 1;
-  const int RESPONSE_COMPLETE = 1;
-  const int RESPONSE_ERROR = 2;
+  static const int LIST_FILE = 0;
+  static const int LIST_DIRECTORY = 1;
+  static const int LIST_LINK = 2;
+  static const int LIST_ERROR = 3;
+  static const int LIST_DONE = 4;
+
+  static const int RESPONSE_TYPE = 0;
+  static const int RESPONSE_PATH = 1;
+  static const int RESPONSE_COMPLETE = 1;
+  static const int RESPONSE_ERROR = 2;
 
   final String path;
   final bool recursive;