Specs: change how .where() works so that it won't leak once its own listeners are removed (e.g. by …

sky/specs/events.md

 Sky Event Model
 ===============
 
 ```dart
 SKY MODULE
 <!-- part of dart:sky -->
 
 <script>
 import 'dart:collection';
 import 'dart:async';
@@ skipping 57 matching lines @@
       try {
         item.b(data);
       } catch (exception, stackTrace) {
         exceptions.add(exception, stackTrace);
       }
     });
     if (exceptions.length > 0)
       throw exceptions;
   }
 
-  Dispatcher<T> where(Filter<T> filter) {
-    var subdispatcher = new Dispatcher<T>();
-    listen((T data) {
-      if (filter(data))
-        subdispatcher._add(data);
-    });
-    return subdispatcher;
-  }
+  Dispatcher<T> where(Filter<T> filter) => new WhereDispatcher<T>(this, filter);
 
   Dispatcher<T> until(Filter<T> filter) {
     var subdispatcher = new Dispatcher<T>();
     Handler handler;
     handler = (T data) {
       if (filter(data))
         unlisten(handler);
       else
         subdispatcher._add(data);
     };
     listen(handler);
     return subdispatcher;
   }
 
   Future<T> firstWhere(Filter<T> filter) {
     Completer completer = new Completer();
     Handler handler;
     handler = (T data) {
       if (filter(data)) {
         completer.complete(data);
         unlisten(handler);
       }
     };
     listen(handler);
     return completer.future;
   }
 }
 
+class WhereDispatcher<T> extends Dispatcher {
+  WhereDispatcher(this.parent, this.filter) : super();
+  Dispatcher parent;
+  Filter filter;
+
+  void listen(Handler<T> handler) {
+    if (_listeners == null || _listeners.length == 0)
+      parent.listen(_handler);
+    super.listen(handler);
+  }
+  bool unlisten(Handler<T> handler) {
+    var result = super.unlisten(handler);
+    if (result && _listeners.length == 0)
+      parent.unlisten(_handler);
+    return result;
+  }
+  void _handler(T data) {
+    if (filter(data))
+      _add(data);
+  }
+}
+
 abstract class Event<ReturnType> {
   Event() { init(); }
   void init() { }
 
   bool get bubbles;
 
   EventTarget _target;
   EventTarget get target => _target;
 
   EventTarget _currentTarget;
@@ skipping 55 matching lines @@
     return event.result;
   }
 
   void _dispatchEventLocally(Event event) {
     event._currentTarget = this;
     _eventsController.add(event);
   }
 }
 </script>
 ```