Specs: Implement the Dispatcher classes.

sky/specs/events.md

 Sky Event Model
 ===============
 
 ```dart
 SKY MODULE
 <!-- part of sky:core -->
 
 <script>
+import 'dart:collection';
+import 'dart:async';
+
+class ExceptionAndStackTrace<T> {
+  const ExceptionAndStackTrace(this.exception, this.stackTrace);
+  final T exception;
+  final StackTrace stackTrace;
+}
+
+class ExceptionListException<T> extends IterableMixin<ExceptionAndStackTrace<T>> implements Exception {
+  List<ExceptionAndStackTrace<T>> _exceptions;
+  void add(T exception, [StackTrace stackTrace = null]) {
+    if (_exceptions == null)
+      _exceptions = new List<ExceptionAndStackTrace<T>>();
+    _exceptions.add(new ExceptionAndStackTrace<T>(exception, stackTrace));
+  }
+  int get length => _exceptions == null ? 0 : _exceptions.length;
+  Iterator<ExceptionAndStackTrace<T>> get iterator => _exceptions.iterator;
+}
+
+typedef bool Filter<T>(T t);
+typedef void Handler<T>(T t);
+
+class DispatcherController<T> {
+  DispatcherController() : dispatcher = new Dispatcher<T>();
+  final Dispatcher<T> dispatcher;
+  void add(T data) => dispatcher._add(data);
+}
+
+class Pair<A, B> {
+  const Pair(this.a, this.b);
+  final A a;
+  final B b;
+}
+
+class Dispatcher<T> {
+  List<Pair<Handler, ZoneUnaryCallback>> _listeners;
+  void listen(Handler<T> handler) {
+    // you should not throw out of this handler
+    if (_listeners == null)
+      _listeners = new List<Pair<Handler, ZoneUnaryCallback>>();
+    _listeners.add(new Pair<Handler, ZoneUnaryCallback>(handler, Zone.current.bindUnaryCallback(handler)));
+  }
+  bool unlisten(Handler<T> handler) {
+    if (_listeners == null)
+      return false;
+    var target = _listeners.lastWhere((v) => v.a == handler, orElse: () => null);
+    if (target == null)
+      return false;
+    _listeners.remove(target);
+    return true;
+  }
+  void _add(T data) {
+    if (_listeners == null)
+      return;
+    ExceptionListException exceptions = new ExceptionListException();
+    // we make a copy of the list here so that the listeners can
+    // mutate our list without worry
+    _listeners.toList().forEach((Pair<Handler, ZoneUnaryCallback> item) {
+      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> 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;
+  }
+}
+
 abstract class Event<ReturnType> {
   Event() { init(); }
   void init() { }
 
   bool get bubbles;
 
   EventTarget _target;
   EventTarget get target => _target;
 
   EventTarget _currentTarget;
   EventTarget get currentTarget => _currentTarget;
 
   bool handled; // precise semantics depend on the event type, but in general, set this when you set result
   ReturnType result;
 
   bool resultIsCompatible(dynamic candidate) => candidate is ReturnType;
 
   // TODO(ianh): abstract API for doing things at shadow tree boundaries 
   // TODO(ianh): do events get blocked at scope boundaries, e.g. focus events when both sides are in the scope?
   // TODO(ianh): do events get retargetted, e.g. focus when leaving a custom element?
 }
 
 class EventTarget {
-  EventTarget() : _eventsController = new DispatcherController<@nonnull Event>();
+  EventTarget() : _eventsController = new DispatcherController<Event>();
 
   Dispatcher get events => _eventsController.dispatcher;
   EventTarget parentNode;
 
-  List<@nonnull EventTarget> getEventDispatchChain() {
+  List<EventTarget> getEventDispatchChain() {
     if (this.parentNode == null) {
       return [this];
     } else {
       var result = this.parentNode.getEventDispatchChain();
       result.insert(0, this);
       return result;
     }
   }
 
   final DispatcherController _eventsController;
 
-  dynamic dispatchEvent(@nonnull Event event, { dynamic defaultResult: null }) { // O(N*M) where N is the length of the chain and M is the average number of listeners per link in the chain
+  dynamic dispatchEvent(Event event, { dynamic defaultResult: null }) { // O(N*M) where N is the length of the chain and M is the average number of listeners per link in the chain
     // note: this will throw an ExceptionListException<ExceptionListException> if any of the listeners threw
     assert(event != null); // event must be non-null
     event.handled = false;
     assert(event.resultIsCompatible(defaultResult));
     event.result = defaultResult;
     event._target = this;
     var chain;
     if (event.bubbles)
       chain = this.getEventDispatchChain();
     else
       chain = [this];
     var exceptions = new ExceptionListException<ExceptionListException>();
     for (var link in chain) {
       try {
         link._dispatchEventLocally(event);
       } on ExceptionListException catch (e) {
         exceptions.add(e);
       }
     }
     if (exceptions.length > 0)
       throw exceptions;
     return event.result;
   }
 
-  void _dispatchEventLocally(@nonnull Event event) {
+  void _dispatchEventLocally(Event event) {
     event._currentTarget = this;
     _eventsController.add(event);
   }
 }
 </script>
 ```