Add native extensions for the Dart shell to mac and windows

runtime/bin/extensions.h

 // 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.
 
 #ifndef BIN_EXTENSIONS_H_
 #define BIN_EXTENSIONS_H_
 
 #include "include/dart_api.h"
 #include "platform/globals.h"
 
 class Extensions {
  public:
-  // TODO(whesse): Make loading extensions lazy, so dynamic library is loaded
-  // only when first native function is called.
   // TODO(whesse): Make extension load from a relative path relative to
   // the library it is in.  Currently loads from current working directory.
   static Dart_Handle LoadExtension(const char* extension_url,
-                                   Dart_Handle library);
+                                   Dart_Handle parent_library);
+
  private:
+  // The returned string must be freed.
+  static char* Concatenate(const char** strings);
+
+  // Platform-specific implementations.
+  static void* LoadExtensionLibrary(const char* library_name);
+  static void* ResolveSymbol(void* lib_handle, const char* symbol);
+
   DISALLOW_ALLOCATION();
   DISALLOW_IMPLICIT_CONSTRUCTORS(Extensions);
 };
 
 #endif  // BIN_EXTENSIONS_H_