samples/poppopwin: upgrade to first-class sample

samples/third_party/pop-pop-win/app_package/platform_app.dart

-library ppw_platform_web;
-
-import 'dart:async';
-
-import 'package:poppopwin/platform_target.dart';
-import 'package:chrome_gen/gen/storage.dart';
-
-class PlatformApp extends PlatformTarget {
-  final StreamController _aboutController = new StreamController(sync: true);
-  bool _about = false;
-
-  PlatformApp() : super.base();
-
-  @override
-  Future clearValues() => storage.local.clear();
-
-  @override
-  Future setValue(String key, String value) =>
-      storage.local.set({key : value});
-
-  @override
-  Future<String> getValue(String key) => storage.local.get(key)
-        .then((Map<String, String> values) => values[key]);
-
-  bool get renderBig => false;
-
-  bool get showAbout => _about;
-
-  Stream get aboutChanged => _aboutController.stream;
-
-  void toggleAbout([bool value]) {
-    assert(_about != null);
-    if(value == null) {
-      value = !_about;
-    }
-    _about = value;
-    _aboutController.add(null);
-  }
-}