fix inconsistent firing of events for extension fields

lib/src/protobuf/readonly_message.dart

 // Copyright (c) 2015, 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.
 
 part of protobuf;
 
 final _emptyList = new List.unmodifiable([]);
 
 /// Modifies a GeneratedMessage so that it's read-only.
 abstract class ReadonlyMessageMixin {
   static final _emptyUnknownFields = new _ReadonlyUnknownFieldSet();
 
   BuilderInfo get info_;
 
   get unknownFields => _emptyUnknownFields;
 
-  List _getDefaultRepeatedField(int tagNumber, List value) => _emptyList;
+  List _getDefaultRepeatedField(int tagNumber, List value, Extension ext) =>
+      _emptyList;
 
   void addExtension(Extension extension, var value) =>
       _readonly("addExtension");
 
   void clear() => _readonly("clear");
 
   void clearExtension(Extension extension) => _readonly("clearExtension");
 
   void clearField(int tagNumber) => _readonly("clearField");
 
@@ skipping 63 matching lines @@
   UnknownFieldSetField _getField(int number) {
     _readonly("a merge method");
     return null; // not reached
   }
 
   void _readonly(String methodName) {
     throw new UnsupportedError(
         "attempted to call $methodName on a read-only UnknownFieldSet");
   }
 }