Menu in StocksApp should dismiss when tapping elsewhere

sky/framework/components/modal_overlay.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 '../fn.dart';
+
+class ModalOverlay extends Component {
+  static final Style _style = new Style('''
+    position: absolute;
+    top: 0;
+    left: 0;
+    bottom: 0;
+    right: 0;''');
+
+  List<Node> children;
+  GestureEventListener onDismiss;
+
+  ModalOverlay({ Object key, this.children, this.onDismiss }) : super(key: key);
+
+  Node build() {
+    return new EventTarget(
+      new Container(
+        style: _style,
+        children: children),
+      onGestureTap: onDismiss);
+  }
+}