Remove custom elements examples, they are no longer maintained

sky/framework/elements/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 attributes="level:number">
@@ skipping 19 matching lines @@
       bottom: 0;
     }
   </style>
   <div id="mask" />
   <div id="content">
     <content/>
   </div>
 </template>
 <script>
 import "../animation/curves.dart";
-import "../animation/timer.dart";
 import "animation/controller.dart";
+import "animation/timer.dart";
 import "dart:math" as math;
 import "dart:sky";
 
 const double _kWidth = 256.0;
 const double _kMinFlingVelocity = 0.4;
 const double _kMinAnimationDurationMS = 246.0;
 const double _kMaxAnimationDurationMS = 600.0;
 const Cubic _kAnimationCurve = easeOut;
 
 @Tagname('sky-drawer')
@@ skipping 105 matching lines @@
       begin: currentPosition,
       end: targetPosition,
       duration: duration,
       curve: linear);
   }
 }
 
 _init(script) => register(script, SkyDrawer);
 </script>
 </sky-element>