[windows] Make most file_win.cc functions use malloc for string conversions.

runtime/bin/file_system_watcher_win.cc

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #if !defined(DART_IO_DISABLED)
 
 #include "platform/globals.h"
 #if defined(TARGET_OS_WINDOWS)
 
 #include "bin/file_system_watcher.h"
@@ skipping 22 matching lines @@
 void FileSystemWatcher::Close(intptr_t id) {
   USE(id);
 }
 
 
 intptr_t FileSystemWatcher::WatchPath(intptr_t id,
                                       const char* path,
                                       int events,
                                       bool recursive) {
   USE(id);
-  const wchar_t* name = StringUtilsWin::Utf8ToWide(path);
-  HANDLE dir = CreateFileW(name,
+  Utf8ToWideScope name(path);
+  HANDLE dir = CreateFileW(name.wide(),
                            FILE_LIST_DIRECTORY,
                            FILE_SHARE_READ |
                                FILE_SHARE_WRITE |
                                FILE_SHARE_DELETE,
                            NULL,
                            OPEN_EXISTING,
                            FILE_FLAG_BACKUP_SEMANTICS | FILE_FLAG_OVERLAPPED,
                            NULL);
 
   if (dir == INVALID_HANDLE_VALUE) {
@@ skipping 81 matching lines @@
   }
   return events;
 }
 
 }  // namespace bin
 }  // namespace dart
 
 #endif  // defined(TARGET_OS_WINDOWS)
 
 #endif  // !defined(DART_IO_DISABLED)