Don't hardcode the list of events types in fn

sky/examples/fn/lib/event.dart

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+part of fn;
+
+class EventHandler {
+  final String type;
+  final sky.EventListener listener;
+
+  EventHandler(this.type, this.listener);
+}
+
+class EventMap {
+  final List<EventHandler> _handlers = new List<EventHandler>();
+
+  void listen(String type, sky.EventListener listener) {
+    assert(listener != null);
+    _handlers.add(new EventHandler(type, listener));
+  }
+
+  void addAll(EventMap events) {
+    _handlers.addAll(events._handlers);
+  }
+}