Allow for no embedder provided library tag handler

runtime/vm/service.h

 // Copyright (c) 2013, 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 VM_SERVICE_H_
 #define VM_SERVICE_H_
 
 #include "include/dart_api.h"
 
 #include "vm/allocation.h"
@@ skipping 59 matching lines @@
   static EmbedderServiceHandler* FindRootEmbedderHandler(const char* name);
   static Dart_Handle GetSource(const char* name);
   static Dart_Handle LibraryTagHandler(Dart_LibraryTag tag, Dart_Handle library,
                                        Dart_Handle url);
   static void SendEvent(intptr_t eventId, const String& eventMessage);
 
   static EmbedderServiceHandler* isolate_service_handler_head_;
   static EmbedderServiceHandler* root_service_handler_head_;
 
   static Isolate* service_isolate_;
-  static Dart_LibraryTagHandler default_handler_;
+  static Dart_LibraryTagHandler embedder_provided_handler_;
   static Dart_Port port_;
   static uint32_t event_mask_;
 };
 
 }  // namespace dart
 
 #endif  // VM_SERVICE_H_