Rationalize Dart mojo and sky package structure

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<UINode> children;
-  GestureEventListener onDismiss;
-
-  ModalOverlay({ Object key, this.children, this.onDismiss }) : super(key: key);
-
-  UINode build() {
-    return new EventListenerNode(
-      new Container(
-        style: _style,
-        children: children),
-      onGestureTap: onDismiss);
-  }
-}