Rationalize Dart mojo and sky package structure

sky/framework/components/button.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';
-import 'ink_well.dart';
-import 'material.dart';
-
-class Button extends Component {
-  static final Style _style = new Style('''
-    -webkit-user-select: none;
-    height: 36px;
-    min-width: 64px;
-    padding: 0 8px;
-    margin: 4px;
-    border-radius: 2px;'''
-  );
-
-  UINode content;
-  int level;
-
-  Button({ Object key, this.content, this.level }) : super(key: key);
-
-  UINode build() {
-    return new StyleNode(
-      new Material(
-        content: new InkWell(children: [content]),
-        level: level),
-      _style);
-  }
-}