Allow constants as field initial values as well as creation thunks

lib/src/protobuf/extension.dart

 // 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.
 
 part of protobuf;
 
 /**
  * An object representing an extension field.
  */
 class Extension extends FieldInfo {
 
   final String extendee;
 
   Extension(this.extendee, String name, int tagNumber, int fieldType,
-            [MakeDefaultFunc makeDefault,
+            [dynamic defaultOrMaker,
             CreateBuilderFunc subBuilder,
             ValueOfFunc valueOf]) :
-     super(name, tagNumber, fieldType, makeDefault, subBuilder, valueOf);
+     super(name, tagNumber, fieldType, defaultOrMaker, subBuilder, valueOf);
 
   int get hashCode => extendee.hashCode * 31 + tagNumber;
 
   bool operator ==(other) {
     if (other is! Extension) return false;
 
     Extension o = other;
     return extendee == o.extendee && tagNumber == o.tagNumber;
   }
 }
-