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

lib/file_generator.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.
 
 part of protoc;
 
 class FileGenerator implements ProtobufContainer {
   final FileDescriptorProto _fileDescriptor;
   final ProtobufContainer _parent;
   final GenerationContext _context;
@@ skipping 98 matching lines @@
     }
 
     String libraryName = _generateLibraryName(filePath);
 
     out.println(
       '///\n'
       '//  Generated code. Do not modify.\n'
       '///\n'
       'library $libraryName;\n'
       '\n'
-      "import 'dart:typed_data';\n\n"
+      "import 'dart:typed_data';\n"
+      '\n'
+      "import 'package:fixnum/fixnum.dart';\n"
       "import 'package:protobuf/protobuf.dart';"
     );
 
     for (String import in _fileDescriptor.dependency) {
       Uri importPath = new Uri.file(import);
       if (importPath.isAbsolute) {
         // protoc should never generate an import with an absolute path.
         throw("FAILURE: Import with absolute path is not supported");
       }
       // Create a relative path from the current file to the import.
@@ skipping 41 matching lines @@
       <String, ProtobufContainer>{};
 
   GenerationContext(this.options);
 
   void register(ProtobufContainer container) {
     _registry[container.fqname] = container;
   }
 
   ProtobufContainer operator [](String fqname) => _registry[fqname];
 }