Prepare for removal of source_filter.gypi:

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"
 
 void* Extensions::LoadExtensionLibrary(const char* library_path,
                                        const char* extension_name) {
   const char* strings[] = { library_path, "/", extension_name, ".dll", NULL };
   char* library_file = Concatenate(strings);
   wchar_t* unicode_library_file = StringUtils::Utf8ToWide(library_file);
   void* lib_handle = LoadLibraryW(unicode_library_file);
   free(unicode_library_file);
   free(library_file);
   return lib_handle;
 }
 
 void* Extensions::ResolveSymbol(void* lib_handle, const char* symbol) {
   return GetProcAddress(reinterpret_cast<HMODULE>(lib_handle), symbol);
 }
+
+#endif  // defined(TARGET_OS_WINDOWS)