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

test/unknown_field_set_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 unknown_field_set_test;
 
 import 'package:protobuf/protobuf.dart';
 import 'package:unittest/unittest.dart';
 
@@ skipping 51 matching lines @@
     expect(optionalInt32.varints[0], expect64(testAllTypes.optionalInt32));
   });
 
   test('testFixed32', () {
     UnknownFieldSetField optionalFixed32 = getField('optionalFixed32');
     expect(optionalFixed32.fixed32s[0], testAllTypes.optionalFixed32);
   });
 
   test('testFixed64', () {
     UnknownFieldSetField optionalFixed64 = getField('optionalFixed64');
-    expect(optionalFixed64.fixed64s[0].getUint32(0),
-           testAllTypes.optionalFixed64.getUint32(0));
-    expect(optionalFixed64.fixed64s[0].getUint32(4),
-           testAllTypes.optionalFixed64.getUint32(4));
+    expect(optionalFixed64.fixed64s[0], testAllTypes.optionalFixed64);
   });
 
   test('testLengthDelimited', () {
     UnknownFieldSetField optionalBytes = getField('optionalBytes');
     expect(optionalBytes.lengthDelimited[0], testAllTypes.optionalBytes);
   });
 
   test('testGroup', () {
     int tagNumberA = new TestAllTypes_OptionalGroup().getTagNumber('a');
     expect(tagNumberA != null, isTrue);
@@ skipping 243 matching lines @@
         ..addField(1, new UnknownFieldSetField()..addLengthDelimited([1, 2]));
     UnknownFieldSet f2 = new UnknownFieldSet()
         ..addField(1, new UnknownFieldSetField()..addLengthDelimited([2, 1]));
 
     _checkEqualsIsConsistent(f1);
     _checkEqualsIsConsistent(f2);
 
     _checkNotEqual(f1, f2);
   });
 }