samples/poppopwin: upgrade to first-class sample

samples/third_party/pop-pop-win/readme.md

-![Pop, Pop, Win!](https://raw.github.com/dart-lang/pop-pop-win/master/web/images/icon_200.png)
-# Pop, Pop, Win!
-
-An implementation of
-<a href="http://en.wikipedia.org/wiki/Minesweeper_(video_game)">Minesweeper</a>
-in [Dart](http://www.dartlang.org).
-
-[![](https://drone.io/kevmoo/pop-pop-win/status.png)](https://drone.io/kevmoo/pop-pop-win/latest)
-
-## Play
-
- * [dart-lang.github.io/pop-pop-win](http://dart-lang.github.io/pop-pop-win/)
- * This is a version compiled to Javascript and should work in any modern browser.
- * This version does not work in Dartium (The custom Chromium version for Dart). The associated code files are not deployed.
-
-## Interaction
-
- * Click on hidden squares to reveal them.
- * Shift-click on squares to flag them.
- * Shift-click on revealed numbers to go fast--but be careful. If you've flagged incorrectly, you'll hit a bomb.
-
-## Developers
- * [Kevin Moore](https://github.com/kevmoo) ([@kevmoo](http://twitter.com/kevmoo))
- * _You? File bugs. Fix bugs. Add features._
-
-## Assets
- * Art - Pete Parisi, [fuzzycube software](http://www.fuzzycubesoftware.com/)
- * Sound Effects - Alistair Hirst, [OMNI Audio](http://omniaudio.com/)
- * Winning Sax Riff - Brian Moore
-
-## License
-
-    Copyright (c) 2012, Pop, Pop, Win! authors
-
-    Licensed under the Apache License, Version 2.0 (the "License");
-    you may not use this file except in compliance with the License.
-    You may obtain a copy of the License at
-
-       http://www.apache.org/licenses/LICENSE-2.0
-
-    Unless required by applicable law or agreed to in writing, software
-    distributed under the License is distributed on an "AS IS" BASIS,
-    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-    See the License for the specific language governing permissions and
-    limitations under the License.