Refactor native extension shared library lookup.

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"
 
 
 namespace dart {
 namespace bin {
 
 class Extensions {
  public:
   // 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,
+  static Dart_Handle LoadExtension(const char* extension_directory,
+                                   const char* extension_filename,
+                                   const char* extension_name,
                                    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_path,
-                                    const char* extension_name);
+  static void* LoadExtensionLibrary(const char* library_file);
   static void* ResolveSymbol(void* lib_handle, const char* symbol);
 
   DISALLOW_ALLOCATION();
   DISALLOW_IMPLICIT_CONSTRUCTORS(Extensions);
 };
 
 }  // namespace bin
 }  // namespace dart
 
 #endif  // BIN_EXTENSIONS_H_