Improve the encoding/decoding to/from system encoding on Windows

runtime/bin/extensions_win.cc

 // Copyright (c) 2012, 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.
 
 #include "platform/globals.h"
 #if defined(TARGET_OS_WINDOWS)
 
 #include "bin/extensions.h"
 #include "bin/utils.h"
+#include "bin/utils_win.h"
 
 
 namespace dart {
 namespace bin {
 
 void* Extensions::LoadExtensionLibrary(const char* library_file) {
-  return LoadLibraryW(StringUtils::Utf8ToWide(library_file));
+  return LoadLibraryW(StringUtilsWin::Utf8ToWide(library_file));
 }
 
 void* Extensions::ResolveSymbol(void* lib_handle, const char* symbol) {
   return GetProcAddress(reinterpret_cast<HMODULE>(lib_handle), symbol);
 }
 
 }  // namespace bin
 }  // namespace dart
 
 #endif  // defined(TARGET_OS_WINDOWS)