Add a shadow to the sky-drawer

sky/framework/sky-drawer.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="shadow.sky" as="shadow" />
 <import src="sky-element.sky" />
 <import src="sky-scrollable.sky" />
 
-<sky-element>
+<sky-element attributes="level:number">
 <template>
   <style>
     #mask {
       background-color: black;
       will-change: opacity;
       position: absolute;
       top: 0;
       left: 0;
       bottom: 0;
       right: 0;
@@ skipping 36 matching lines @@
   SkyDrawer() {
     _animation = new AnimationController(this);
 
     addEventListener('pointerdown', _handlePointerDown);
     addEventListener('pointermove', _handlePointerMove);
     addEventListener('pointerup', _handlePointerUp);
     addEventListener('pointercancel', _handlePointerCancel);
   }
 
   void shadowRootReady() {
+    shadow.applyTo(shadowRoot);
     _mask = shadowRoot.getElementById('mask');
     _mask.addEventListener('gesturetap', _handleMaskTap);
     _content = shadowRoot.getElementById('content');
     _content.addEventListener('gestureflingstart', _handleFlingStart);
     position = -_kWidth;
   }
 
   void toggle() {
     if (isMostlyClosed)
       open();
@@ skipping 78 matching lines @@
       begin: currentPosition,
       end: targetPosition,
       duration: duration,
       curve: linear);
   }
 }
 
 _init(script) => register(script, SkyDrawer);
 </script>
 </sky-element>