Add native extensions for the Dart shell to mac and windows

runtime/bin/extensions_macos.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 "bin/extensions.h"
+#include <dlfcn.h>
 
-#include "include/dart_api.h"
-#include "platform/assert.h"
+void* Extensions::LoadExtensionLibrary(const char* library_name) {
+  char* library_path = MakeString("./%s.so", library_name);
+  void* lib_handle = dlopen(library_path, RTLD_LAZY);
+  delete[] library_path;
+  return lib_handle;
+}
 
-Dart_Handle Extensions::LoadExtension(const char* extension_url,
-                                      Dart_Handle parent_library) {
-  UNIMPLEMENTED();
-  return NULL;
+void* Extensions::ResolveSymbol(void* lib_handle, const char* symbol) {
+  void* result = dlsym(lib_handle, symbol);
+  if (dlerror() != NULL) return NULL;
+  return result;
 }