FilePath::Append() doesn't allow you to append a fully-qualified...

net/tools/dump_cache/cache_dumper.cc

 // Copyright (c) 2009 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "net/tools/dump_cache/cache_dumper.h"
 
 #include "net/base/io_buffer.h"
 #include "net/disk_cache/entry_impl.h"
 #include "net/http/http_cache.h"
 #include "net/http/http_response_headers.h"
@@ skipping 52 matching lines @@
 
 bool DiskDumper::CreateEntry(const std::string& key,
                              disk_cache::Entry** entry) {
   FilePath path(path_);
   entry_path_ = net::UrlToFilenameEncoder::Encode(key, path);
 
 #ifdef WIN32_LARGE_FILENAME_SUPPORT
   // In order for long filenames to work, we'll need to prepend
   // the windows magic token.
   const std::wstring kLongFilenamePrefix(L"\\\\?\\");
-  entry_path_ = FilePath(kLongFilenamePrefix).Append(entry_path_);
+  // There is no way to prepend to a filename.  We simply *have*
+  // to convert to a wstring to do this.
+  std::wstring name = kLongFilenamePrefix;
+  name.append(entry_path_.ToWStringHack());
+  entry_path_ = FilePath(name);
 #endif
 
   entry_url_ = key;
 
   FilePath directory = entry_path_.DirName();
   SafeCreateDirectory(directory.value());
 
   std::wstring file = entry_path_.value();
 #ifdef WIN32_LARGE_FILENAME_SUPPORT
   entry_ = CreateFileW(file.c_str(), GENERIC_WRITE|GENERIC_READ, 0, 0,
@@ skipping 102 matching lines @@
 
 void DiskDumper::CloseEntry(disk_cache::Entry* entry, base::Time last_used,
                           base::Time last_modified) {
 #ifdef WIN32_LARGE_FILENAME_SUPPORT
   CloseHandle(entry_);
 #else
   file_util::CloseFile(entry_);
 #endif
 }