Add native extensions for the Dart shell to the MacOs platform

runtime/bin/extensions_posix.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 <stdio.h>
 #include <dlfcn.h>
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "platform/globals.h"
 #include "bin/extensions.h"
 #include "bin/dartutils.h"
 
 Dart_Handle Extensions::LoadExtension(const char* extension_url,
                                       Dart_Handle parent_library) {
   // TODO(whesse): Consider making loading extensions lazy, so the
   // dynamic library is loaded only when first native function is called.
   ASSERT(DartUtils::IsDartExtensionSchemeURL(extension_url));
   const char* library_name =
       extension_url + strlen(DartUtils::kDartExtensionScheme);
   if (strchr(library_name, '/') != NULL) {
     return Dart_Error("path components not allowed in extension library name");
   }
   const int buffer_length = strlen(library_name) + strlen("./lib.so") + 1;

[Ivan Posva, 2012/03/05 23:07:47]

Please do not share Linux and Mac OS X code. There is a huge urge to make them
common until you realize it does not work. Prime example here: Shared libraries
on Mac OS X do not end with .so. Siva and I have been down that path many times
and we did decide on OS-specific files for a reason.

   char* library_path = new char[buffer_length];
   snprintf(library_path, buffer_length, "./lib%s.so", library_name);
 
   void* lib_handle = dlopen(library_path, RTLD_LAZY);
   if (!lib_handle) {
     delete[] library_path;
     return Dart_Error("cannot find extension library");
   }
   // Reuse library_path buffer for intialization function name.
   char* library_init_function = library_path;
   snprintf(library_init_function, buffer_length, "%s_Init", library_name);
   typedef Dart_Handle (*InitFunctionType)(Dart_Handle import_map);
   InitFunctionType fn = reinterpret_cast<InitFunctionType>(
       dlsym(lib_handle, library_init_function));
   delete[] library_path;
   char *error = dlerror();
   if (error != NULL) {
     return Dart_Error(error);
   }
   return (*fn)(parent_library);
 }