diff -r 000000000000 -r 6474c204b198 dom/file/MemoryStreams.cpp
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/dom/file/MemoryStreams.cpp	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,90 @@
+/* -*- 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/. */
+
+#include "MemoryStreams.h"
+
+#include "nsStreamUtils.h"
+
+USING_FILE_NAMESPACE
+
+// static
+already_AddRefed<MemoryOutputStream>
+MemoryOutputStream::Create(uint64_t aSize)
+{
+  NS_ASSERTION(aSize, "Passed zero size!");
+
+  NS_ENSURE_TRUE(aSize <= UINT32_MAX, nullptr);
+
+  nsRefPtr<MemoryOutputStream> stream = new MemoryOutputStream();
+
+  char* dummy;
+  uint32_t length = stream->mData.GetMutableData(&dummy, aSize, fallible_t());
+  NS_ENSURE_TRUE(length == aSize, nullptr);
+
+  return stream.forget();
+}
+
+NS_IMPL_ISUPPORTS(MemoryOutputStream, nsIOutputStream)
+
+NS_IMETHODIMP
+MemoryOutputStream::Close()
+{
+  mData.Truncate(mOffset);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+MemoryOutputStream::Write(const char* aBuf, uint32_t aCount, uint32_t* _retval)
+{
+  return WriteSegments(NS_CopySegmentToBuffer, (char*)aBuf, aCount, _retval);
+}
+
+NS_IMETHODIMP
+MemoryOutputStream::Flush()
+{
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+MemoryOutputStream::WriteFrom(nsIInputStream* aFromStream, uint32_t aCount,
+                              uint32_t* _retval)
+{
+  return NS_ERROR_NOT_IMPLEMENTED;
+}
+
+NS_IMETHODIMP
+MemoryOutputStream::WriteSegments(nsReadSegmentFun aReader, void* aClosure,
+                                  uint32_t aCount, uint32_t* _retval)
+{
+  NS_ASSERTION(mData.Length() >= mOffset, "Bad stream state!");
+
+  uint32_t maxCount = mData.Length() - mOffset;
+  if (maxCount == 0) {
+    *_retval = 0;
+    return NS_OK;
+  }
+
+  if (aCount > maxCount) {
+    aCount = maxCount;
+  }
+
+  nsresult rv = aReader(this, aClosure, mData.BeginWriting() + mOffset, 0,
+                        aCount, _retval);
+  if (NS_SUCCEEDED(rv)) {
+    NS_ASSERTION(*_retval <= aCount,
+                 "Reader should not read more than we asked it to read!");
+    mOffset += *_retval;
+  }
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+MemoryOutputStream::IsNonBlocking(bool* _retval)
+{
+  *_retval = false;
+  return NS_OK;
+}