Add a shadow to the sky-drawer

sky/examples/stocks/stocks.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/framework/sky-drawer-header.sky" />
 <import src="/sky/framework/sky-drawer.sky" />
 <import src="/sky/framework/sky-element.sky" />
 <import src="/sky/framework/sky-icon.sky" />
 <import src="/sky/framework/sky-menu-divider.sky" />
@@ skipping 31 matching lines @@
       margin: 0 4px;
     }
     #title {
       flex: 1;
       margin: 0 4px;
     }
     sky-scrollable {
       flex: 1;
     }
   </style>
-  <sky-drawer id="drawer">
+  <sky-drawer id="drawer" level="3">
     <sky-drawer-header>
       Stocks
     </sky-drawer-header>
     <sky-menu-item icon="content/inbox">Inbox</sky-menu-item>
     <sky-menu-item icon="content/drafts">Drafts</sky-menu-item>
     <sky-menu-divider />
     <sky-menu-item icon="action/settings">Settings</sky-menu-item>
     <sky-menu-item icon="action/help">Help &amp; feedback</sky-menu-item>
   </sky-drawer>
   <sky-toolbar level="2">
@@ skipping 40 matching lines @@
   }
 
   void _handleMenuClick(_) {
     _drawer.toggle();
   }
 }
 
 _init(script) => register(script, Stocks);
 </script>
 </sky-element>