Add native extensions for the Dart shell to the MacOs platform

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
deleted file mode 100644
index d81e670eef10218fe7bc68228faa4d0d88a9083c..0000000000000000000000000000000000000000
--- a/runtime/bin/extensions_macos.cc
+++ /dev/null
@@ -1,14 +0,0 @@
-// 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 "bin/extensions.h"
-
-#include "include/dart_api.h"
-#include "platform/assert.h"
-
-Dart_Handle Extensions::LoadExtension(const char* extension_url,
-                                      Dart_Handle parent_library) {
-  UNIMPLEMENTED();
-  return NULL;
-}