diff --git a/mock_file_writer.h b/mock_file_writer.h
new file mode 100644
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--- /dev/null
+++ b/mock_file_writer.h
@@ -0,0 +1,59 @@
+// Copyright (c) 2009 The Chromium OS Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef UPDATE_ENGINE_MOCK_FILE_WRITER_H__
+#define UPDATE_ENGINE_MOCK_FILE_WRITER_H__
+
+#include "base/basictypes.h"
+#include "update_engine/file_writer.h"
+
+// MockFileWriter is an implementation of FileWriter. It will succeed
+// calls to Open(), Close(), but not do any work. All calls to Write()
+// will append the passed data to an internal vector.
+
+namespace chromeos_update_engine {
+
+class MockFileWriter : public FileWriter {
+ public:
+  MockFileWriter() : was_opened_(false), was_closed_(false) {}
+
+  virtual int Open(const char* path, int flags, mode_t mode) {
+    CHECK(!was_opened_);
+    CHECK(!was_closed_);
+    was_opened_ = true;
+    return 0;
+  }
+
+  virtual int Write(const void* bytes, size_t count) {
+    CHECK(was_opened_);
+    CHECK(!was_closed_);
+    const char* char_bytes = reinterpret_cast<const char*>(bytes);
+    bytes_.insert(bytes_.end(), char_bytes, char_bytes + count);
+    return count;
+  }
+
+  virtual int Close() {
+    CHECK(was_opened_);
+    CHECK(!was_closed_);
+    was_closed_ = true;
+    return 0;
+  }
+
+  const std::vector<char>& bytes() {
+    return bytes_;
+  }
+ private:
+  // The internal store of all bytes that have been written
+  std::vector<char> bytes_;
+
+  // These are just to ensure FileWriter methods are called properly.
+  bool was_opened_;
+  bool was_closed_;
+
+  DISALLOW_COPY_AND_ASSIGN(MockFileWriter);
+};
+
+}  // namespace chromeos_update_engine
+
+#endif  // UPDATE_ENGINE_MOCK_FILE_WRITER_H__