Implement quantum ink splashes

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 attributes="level:number">
@@ skipping 126 matching lines @@
     if (!_animation.isAnimating)
       _settle();
   }
 
   void _handlePointerCancel(_) {
     if (!_animation.isAnimating)
       _settle();
   }
 
   void _handleFlingStart(event) {
-    double direction = event.velocityX.sign();
+    double direction = event.velocityX.sign;
     double velocityX = event.velocityX.abs() / 1000;
     if (velocityX < _kMinFlingVelocity)
       return;
     double targetPosition = direction < 0.0 ? -kWidth : 0.0;
     double currentPosition = _position;
     double distance = (targetPosition - currentPosition).abs();
     double duration = distance / velocityX;
     _animation.start(
       begin: currentPosition,
       end: targetPosition,
       duration: duration,
       curve: linear);
   }
 }
 
 _init(script) => register(script, SkyDrawer);
 </script>
 </sky-element>