Add support for scroll and fling gestures to SkyShell.

sky/framework/sky-scrollable.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-element.sky" />
 
 <sky-element>
 <template>
   <style>
@@ skipping 76 matching lines @@
   double get scrollOffset => _scrollOffset;
 
   set scrollOffset(double value) {
     // TODO(abarth): Can we get these values without forcing a synchronous layout?
     double outerHeight = clientHeight.toDouble();
     double innerHeight = _scrollable.clientHeight.toDouble();
     double scrollRange = innerHeight - outerHeight;
     double newScrollOffset = math.max(0.0, math.min(scrollRange, value));
     if (newScrollOffset == _scrollOffset)
       return;
+    // TODO(eseidel): We should scroll in device pixels instead of logical
+    // pixels, but to do that correctly we need to use a device pixel unit.
     _scrollOffset = newScrollOffset;
-    String transform = 'translateY(${(-_scrollOffset).toStringAsFixed(2)}px)';
+    String transform = 'translateY(${(-_scrollOffset).toInt()}px)';
     _scrollable.style['transform'] = transform;
 
     double topPercent = newScrollOffset / innerHeight * 100.0;
     double heightPercent = outerHeight / innerHeight * 100.0;
     _vbar.style['top'] = '${topPercent}%';
     _vbar.style['height'] = '${heightPercent}%';
   }
 
   bool scrollBy(double scrollDelta) {
     double oldScrollOffset = _scrollOffset;
@@ skipping 42 matching lines @@
   }
 
   void _handleWheel(WheelEvent event) {
     scrollBy(-event.offsetY);
   }
 }
 
 _init(script) => register(script, SkyScrollable);
 </script>
 </sky-element>