Rationalize Dart mojo and sky package structure

sky/framework/components/button_base.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';
-
-abstract class ButtonBase extends Component {
-  bool highlight = false;
-
-  ButtonBase({ Object key }) : super(key: key);
-
-  UINode buildContent();
-
-  UINode build() {
-    return new EventListenerNode(
-      buildContent(),
-      onPointerDown: _handlePointerDown,
-      onPointerUp: _handlePointerUp,
-      onPointerCancel: _handlePointerCancel
-    );
-  }
-
-  void _handlePointerDown(_) {
-    setState(() {
-      highlight = true;
-    });
-  }
-  void _handlePointerUp(_) {
-    setState(() {
-      highlight = false;
-    });
-  }
-  void _handlePointerCancel(_) {
-    setState(() {
-      highlight = false;
-    });
-  }
-}