Rationalize Dart mojo and sky package structure

sky/framework/elements/shadow.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.
--->
-<template>
-  <style>
-  :host([level="1"]) {
-    box-shadow: 0 1px 3px rgba(0,0,0,0.12), 0 1px 2px rgba(0,0,0,0.24);
-  }
-  :host([level="2"]) {
-    box-shadow: 0 3px 6px rgba(0,0,0,0.16), 0 3px 6px rgba(0,0,0,0.23);
-  }
-  :host([level="3"]) {
-    box-shadow: 0 10px 20px rgba(0,0,0,0.19), 0 6px 6px rgba(0,0,0,0.23);
-  }
-  :host([level="4"]) {
-    box-shadow: 0 14px 28px rgba(0,0,0,0.25), 0 10px 10px rgba(0,0,0,0.22);
-  }
-  :host([level="5"]) {
-    box-shadow: 0 19px 38px rgba(0,0,0,0.30), 0 15px 12px rgba(0,0,0,0.22);
-  }
-  </style>
-</template>
-<script>
-import "dart:sky";
-
-HTMLStyleElement _kStyleElement;
-
-void applyTo(ShadowRoot shadowRoot) {
-  shadowRoot.appendChild(_kStyleElement.cloneNode(deep: true));
-}
-
-_init(script) {
-  HTMLTemplateElement template = script.owner.querySelector('template');
-  _kStyleElement = template.content.querySelector('style');
-}
-</script>
-</sky-element>