Update the protocol buffer compiler plugin to use Int64 from the fixnum package

test/generated_message_test.dart

 #!/usr/bin/env dart
 // 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.
 
 library generated_message_test;
 
 import 'package:protobuf/protobuf.dart';
 import 'package:unittest/unittest.dart';
 
 import '../out/protos/google/protobuf/unittest.pb.dart';
 import '../out/protos/google/protobuf/unittest_import.pb.dart';
 import '../out/protos/google/protobuf/unittest_optimize_for.pb.dart';
 import '../out/protos/multiple_files_test.pb.dart';
 import '../out/protos/nested_extension.pb.dart';
 import '../out/protos/non_nested_extension.pb.dart';
 
 import 'test_util.dart';
 
 void main() {
   final throwsInvalidProtocolBufferException =
       throwsA(new isInstanceOf<InvalidProtocolBufferException>());
-
   test('testProtosShareRepeatedArraysIfDidntChange', () {
     TestAllTypes value1 = new TestAllTypes()
         ..repeatedInt32.add(100)
         ..repeatedImportEnum.add(ImportEnum.IMPORT_BAR)
         ..repeatedForeignMessage.add(new ForeignMessage());
 
     TestAllTypes value2 = value1.clone();
 
     expect(value2.repeatedInt32, value1.repeatedInt32);
     expect(value2.repeatedImportEnum, value1.repeatedImportEnum);
@@ skipping 420 matching lines @@
     SparseEnumMessage message = new SparseEnumMessage()
         ..sparseEnum = TestSparseEnum.SPARSE_E;
     expect(message.sparseEnum.value < 0, isTrue,
         reason: 'enum.value should be -53452');
     SparseEnumMessage message2 =
         new SparseEnumMessage.fromBuffer(message.writeToBuffer());
     expect(message2.sparseEnum, TestSparseEnum.SPARSE_E,
            reason: 'should resolve back to SPARSE_E');
   });
 }