Update fixnum to new package guidelines.

pkg/fixnum/fixnum.dart

Index: pkg/fixnum/fixnum.dart
diff --git a/pkg/fixnum/fixnum.dart b/pkg/fixnum/fixnum.dart
deleted file mode 100644
index 837564cea3693b5a5ae16d9c89a4c92a62ac6898..0000000000000000000000000000000000000000
--- a/pkg/fixnum/fixnum.dart
+++ /dev/null
@@ -1,9 +0,0 @@
-// 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.
-
-library fixnum;
-import 'dart:math' as Math;
-part 'intx.dart';
-part 'int32.dart';
-part 'int64.dart';