Add the package implementation.

CONTRIBUTING.md

 Want to contribute? Great! First, read this page (including the small print at
 the end).
 
 ### Before you contribute
 Before we can use your code, you must sign the
 [Google Individual Contributor License Agreement](https://cla.developers.google.com/about/google-individual)
 (CLA), which you can do online. The CLA is necessary mainly because you own the
 copyright to your changes, even after your contribution becomes part of our
 codebase, so we need your permission to use and distribute your code. We also
 need to be sure of various other things—for instance that you'll tell us if you
 know that your code infringes on other people's patents. You don't have to sign
 the CLA until after you've submitted your code for review and a member has
 approved it, but you must do it before we can put your code into our codebase.
 
 Before you start working on a larger contribution, you should get in touch with
 us first through the issue tracker with your idea so that we can help out and
 possibly guide you. Coordinating up front makes it much easier to avoid
 frustration later on.
 
 ### Code reviews
 All submissions, including submissions by project members, require review.
 
 ### File headers
 All files in the project must start with the following header.
 
-    // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
+    // Copyright (c) 2016, 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.
 
 ### The small print
 Contributions made by corporations are covered by a different agreement than the
 one above, the
 [Software Grant and Corporate Contributor License Agreement](https://developers.google.com/open-source/cla/corporate).