Add a shadow to the sky-drawer

sky/framework/sky-drawer.sky

Index: sky/framework/sky-drawer.sky
diff --git a/sky/framework/sky-drawer.sky b/sky/framework/sky-drawer.sky
index a772d9874bbc511dc5be9f177a896ef2e16e27a8..d9a4e9ab7f8efc8eb1d8b6a9439b675669584ec6 100644
--- a/sky/framework/sky-drawer.sky
+++ b/sky/framework/sky-drawer.sky
@@ -3,10 +3,11 @@
 // 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 {
@@ -63,6 +64,7 @@ class SkyDrawer extends SkyElement implements AnimationDelegate {
   }
 
   void shadowRootReady() {
+    shadow.applyTo(shadowRoot);
     _mask = shadowRoot.getElementById('mask');
     _mask.addEventListener('gesturetap', _handleMaskTap);
     _content = shadowRoot.getElementById('content');