Fix build bot error.

runtime/bin/extensions_win.cc

Index: runtime/bin/extensions_win.cc
diff --git a/runtime/bin/extensions_win.cc b/runtime/bin/extensions_win.cc
index ec68c5ba90f13a755f51618d172df47a6dd6030f..33e3273cd95ab4d235aba404e5df8d7aed18569c 100644
--- a/runtime/bin/extensions_win.cc
+++ b/runtime/bin/extensions_win.cc
@@ -28,6 +28,10 @@ Dart_Handle Extensions::LoadExtensionLibrary(const char* library_file,
   return Dart_Null();
 }
 
+void* Extensions::LoadExtensionLibrary(const char* library_file) {
+  return LoadLibraryW(StringUtilsWin::Utf8ToWide(library_file));
+}
+
 Dart_Handle Extensions::ResolveSymbol(void* lib_handle,
                                       const char* symbol,
                                       void** init_function) {
@@ -41,6 +45,10 @@ Dart_Handle Extensions::ResolveSymbol(void* lib_handle,
   return Dart_Null();
 }
 
+void* Extensions::ResolveSymbol(void* lib_handle, const char* symbol) {
+  return GetProcAddress(reinterpret_cast<HMODULE>(lib_handle), symbol);
+}
+
 }  // namespace bin
 }  // namespace dart