Add generic types for strong mode

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 {
+class Extension<T> extends FieldInfo<T> {
   final String extendee;
 
   Extension(this.extendee, String name, int tagNumber, int fieldType,
       [dynamic defaultOrMaker,
       CreateBuilderFunc subBuilder,
       ValueOfFunc valueOf])
       : super(name, tagNumber, null, fieldType, defaultOrMaker, subBuilder,
             valueOf);
 
   Extension.repeated(
       this.extendee, String name, int tagNumber, int fieldType, CheckFunc check,
       [CreateBuilderFunc subBuilder, ValueOfFunc valueOf])
       : super.repeated(
             name, tagNumber, null, fieldType, check, 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;
   }
 }