Fix build bot error.

runtime/bin/extensions_macos.cc

Index: runtime/bin/extensions_macos.cc
diff --git a/runtime/bin/extensions_macos.cc b/runtime/bin/extensions_macos.cc
index 1d6a5f6c321ddb434127c272a3a3a12d8fa0e970..194050f5562d52766adc6dd1f24e638462ccf78c 100644
--- a/runtime/bin/extensions_macos.cc
+++ b/runtime/bin/extensions_macos.cc
@@ -26,6 +26,10 @@ Dart_Handle Extensions::LoadExtensionLibrary(const char* library_file,
   return Dart_Null();
 }
 
+void* Extensions::LoadExtensionLibrary(const char* library_file) {
+  return dlopen(library_file, RTLD_LAZY);
+}
+
 Dart_Handle Extensions::ResolveSymbol(void* lib_handle,
                                       const char* symbol,
                                       void** init_function) {
@@ -39,6 +43,13 @@ Dart_Handle Extensions::ResolveSymbol(void* lib_handle,
   return Dart_Null();
 }
 
+void* Extensions::ResolveSymbol(void* lib_handle, const char* symbol) {
+  dlerror();
+  void* result = dlsym(lib_handle, symbol);
+  if (dlerror() != NULL) return NULL;
+  return result;
+}
+
 }  // namespace bin
 }  // namespace dart