diff -r 000000000000 -r 6474c204b198 dom/file/MemoryStreams.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/dom/file/MemoryStreams.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,46 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_dom_file_memorystreams_h__
+#define mozilla_dom_file_memorystreams_h__
+
+#include "FileCommon.h"
+
+#include "nsIOutputStream.h"
+
+BEGIN_FILE_NAMESPACE
+
+class MemoryOutputStream : public nsIOutputStream
+{
+public:
+  NS_DECL_THREADSAFE_ISUPPORTS
+  NS_DECL_NSIOUTPUTSTREAM
+
+  static already_AddRefed<MemoryOutputStream>
+  Create(uint64_t aSize);
+
+
+  const nsCString&
+  Data() const
+  {
+    return mData;
+  }
+
+private:
+  MemoryOutputStream()
+  : mOffset(0)
+  { }
+
+  virtual ~MemoryOutputStream()
+  { }
+
+  nsCString mData;
+  uint64_t mOffset;
+};
+
+END_FILE_NAMESPACE
+
+#endif // mozilla_dom_file_memorystreams_h__