samples/poppopwin: upgrade to first-class sample

samples/third_party/pop-pop-win/lib/platform_target.dart

-library ppw_platform;
-
-import 'dart:async';
-
-abstract class PlatformTarget {
-  bool _initialized = false;
-
-  factory PlatformTarget() => new _DefaultPlatform();
-
-  PlatformTarget.base();
-
-  bool get initialized => _initialized;
-
-  void initialize() {
-    assert(!_initialized);
-    _initialized = true;
-  }
-
-  Future clearValues();
-
-  Future setValue(String key, String value);
-
-  Future<String> getValue(String key);
-
-  void trackAnalyticsEvent(String category, String action, [String label,
-                                                            int value]) {
-    print('Analytics:: '
-        'category: $category; action: $action; label: $label; value: $value');
-  }
-
-  bool get renderBig;
-
-  bool get showAbout;
-
-  void toggleAbout([bool value]);
-
-  Stream get aboutChanged;
-}
-
-class _DefaultPlatform extends PlatformTarget {
-  final Map<String, String> _values = new Map<String, String>();
-  final StreamController _aboutController = new StreamController(sync: true);
-  bool _about = false;
-
-  _DefaultPlatform() : super.base();
-
-  @override
-  Future clearValues() => new Future(_values.clear);
-
-  @override
-  Future setValue(String key, String value) =>
-      new Future(() { _values[key] = value; });
-
-  @override
-  Future<String> getValue(String key) => new Future(() => _values[key]);
-
-  bool get renderBig => false;
-
-  void toggleAbout([bool value]) {
-    assert(_about != null);
-    if(value == null) {
-      value = !_about;
-    }
-    _about = value;
-    _aboutController.add(null);
-  }
-
-  bool get showAbout => _about;
-
-  Stream get aboutChanged => _aboutController.stream;
-}