samples/poppopwin: upgrade to first-class sample

samples/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 {
@@ skipping 21 matching lines @@
       value = !_about;
     }
     _about = value;
     _aboutController.add(null);
   }
 
   bool get showAbout => _about;
 
   Stream get aboutChanged => _aboutController.stream;
 }