Mark fn2's EventListenerNode work for PointerEvent

Prevously we listened for events on the document and then walked up the
component hierarchy looking for EventListenerNodes. Now we do something similar
by hooking the event dispatching logic in the AppView.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1153343004

examples/fn2/container.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.
+
+import 'dart:sky' as sky;
+import 'package:sky/framework/fn2.dart';
+
+class ContainerApp extends App {
+  UINode build() {
+    return new EventListenerNode(
+      new Rectangle(0xFF00FFFF),
+      onPointerDown: _handlePointerDown);
+  }
+
+  void _handlePointerDown(sky.PointerEvent event) {
+    print("_handlePointerDown");
+  }
+}
+
+void main() {
+  new ContainerApp();
+}