Fixes memory leak of async directory lister

runtime/bin/directory_patch.dart

Index: runtime/bin/directory_patch.dart
diff --git a/runtime/bin/directory_patch.dart b/runtime/bin/directory_patch.dart
index f8643ae30aab326d78e3a30bedbe457b391e0265..f98a52e966e57d77f457de9c00dc2381b9161351 100644
--- a/runtime/bin/directory_patch.dart
+++ b/runtime/bin/directory_patch.dart
@@ -16,3 +16,21 @@ patch class _Directory {
   /* patch */ static List _list(String path, bool recursive, bool followLinks)
       native "Directory_List";
 }
+
+patch class _AsyncDirectoryListerOps {
+  /* patch */ factory _AsyncDirectoryListerOps(int pointer) =>
+      new _AsyncDirectoryListerOpsImpl(pointer);
+}
+
+class _AsyncDirectoryListerOpsImpl extends NativeFieldWrapperClass1
+                                   implements _AsyncDirectoryListerOps {
+  _AsyncDirectoryListerOpsImpl._();
+
+  factory _AsyncDirectoryListerOpsImpl(int pointer)
+      => new _AsyncDirectoryListerOpsImpl._().._setPointer(pointer);
+
+  void _setPointer(int pointer)
+      native "Directory_SetAsyncDirectoryListerPointer";
+  int getPointer()
+      native "Directory_GetAsyncDirectoryListerPointer";
+}