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

test/file_generator_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 file_generator_test;
 
 import 'package:protoc-plugin/src/descriptor.pb.dart';
 import 'package:protoc-plugin/src/plugin.pb.dart';
 import 'package:protoc-plugin/protoc.dart';
@@ skipping 58 matching lines @@
   test('testMessageGenerator', () {
     // NOTE: Below > 80 cols because it is matching generated code > 80 cols.
     String expected = r'''
 ///
 //  Generated code. Do not modify.
 ///
 library test;
 
 import 'dart:typed_data';
 
+import 'package:fixnum/fixnum.dart';
 import 'package:protobuf/protobuf.dart';
 
 class PhoneNumber extends GeneratedMessage {
   static final BuilderInfo _i = new BuilderInfo('PhoneNumber')
     ..a(1, 'number', GeneratedMessage.QS)
     ..a(2, 'type', GeneratedMessage.O3)
     ..a(3, 'name', GeneratedMessage.OS, () => '\$')
   ;
 
   PhoneNumber() : super();
@@ skipping 34 matching lines @@
   test('testMessageGeneratorTopLevelEmun', () {
     // NOTE: Below > 80 cols because it is matching generated code > 80 cols.
     String expected = r'''
 ///
 //  Generated code. Do not modify.
 ///
 library test;
 
 import 'dart:typed_data';
 
+import 'package:fixnum/fixnum.dart';
 import 'package:protobuf/protobuf.dart';
 
 class PhoneType extends ProtobufEnum {
   static const PhoneType MOBILE = const PhoneType._(0, 'MOBILE');
   static const PhoneType HOME = const PhoneType._(1, 'HOME');
   static const PhoneType WORK = const PhoneType._(2, 'WORK');
 
   static const PhoneType BUSINESS = WORK;
 
   static const List<PhoneType> values = const <PhoneType> [
@@ skipping 53 matching lines @@
   test('testMessageGeneratorPackage', () {
     // NOTE: Below > 80 cols because it is matching generated code > 80 cols.
     String expected = r'''
 ///
 //  Generated code. Do not modify.
 ///
 library pb_library;
 
 import 'dart:typed_data';
 
+import 'package:fixnum/fixnum.dart';
 import 'package:protobuf/protobuf.dart';
 
 class PhoneNumber extends GeneratedMessage {
   static final BuilderInfo _i = new BuilderInfo('PhoneNumber')
     ..a(1, 'number', GeneratedMessage.QS)
     ..a(2, 'type', GeneratedMessage.O3)
     ..a(3, 'name', GeneratedMessage.OS, () => '\$')
   ;
 
   PhoneNumber() : super();
@@ skipping 35 matching lines @@
   test('testMessageGeneratorFieldNameOption', () {
     // NOTE: Below > 80 cols because it is matching generated code > 80 cols.
     String expected = r'''
 ///
 //  Generated code. Do not modify.
 ///
 library test;
 
 import 'dart:typed_data';
 
+import 'package:fixnum/fixnum.dart';
 import 'package:protobuf/protobuf.dart';
 
 class PhoneNumber extends GeneratedMessage {
   static final BuilderInfo _i = new BuilderInfo('PhoneNumber')
     ..a(1, 'no', GeneratedMessage.QS)
     ..a(2, 'the_type', GeneratedMessage.O3)
     ..a(3, 'name_', GeneratedMessage.OS, () => '\$')
   ;
 
   PhoneNumber() : super();
@@ skipping 29 matching lines @@
     var options =
         new GenerationOptions(
             request, new CodeGeneratorResponse());
     FileGenerator fg =
         new FileGenerator(
             fd, null, new GenerationContext(options));
     fg.generate(writer);
     expect(buffer.toString(), expected);
   });
 }