Rationalize Dart mojo and sky package structure

sky/framework/elements/sky-checkbox.sky

-<!--
-// 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 src="sky-element.sky" />
-<import src="sky-icon.sky" />
-
-<sky-element attributes="checked:boolean">
-<template>
-  <style>
-    :host {
-      display: inline-flex;
-      -webkit-user-select: none;
-      margin: 8px 16px;
-    }
-  </style>
-  <sky-icon size="18" />
-</template>
-<script>
-import "dart:sky";
-
-const String _kOnIcon = 'toggle/check_box_black';
-const String _kOffIcon = 'toggle/check_box_outline_blank_black';
-
-@Tagname('sky-checkbox')
-class SkyCheckbox extends SkyElement {
-  SkyIcon _icon;
-
-  SkyCheckbox() {
-    addEventListener('click', _handleClick);
-  }
-
-  void shadowRootReady() {
-    _icon = shadowRoot.querySelector('sky-icon');
-    _icon.type = checked ? _kOnIcon : _kOffIcon;
-  }
-
-  void checkedChanged(bool oldValue, bool newValue) {
-    if (_icon != null)
-      _icon.type = newValue ? _kOnIcon : _kOffIcon;
-  }
-
-  void _handleClick(_) {
-    checked = !checked;
-  }
-}
-
-_init(script) => register(script, SkyCheckbox);
-</script>
-</sky-element>