Cleanup events related to material splashes

sky/framework/components/drawer.dart

 // 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 '../animation/animation.dart';
 import '../animation/curves.dart';
 import '../fn.dart';
 import '../theme/colors.dart';
 import '../theme/shadows.dart';
 import 'dart:async';
@@ skipping 16 matching lines @@
     value = -_kWidth;
   }
 
   void toggle(_) => _isMostlyClosed ? _open() : _close();
 
   void handleMaskTap(_) => _close();
 
   void handlePointerDown(_) => stop();
 
   void handlePointerMove(sky.PointerEvent event) {
-    assert(!isAnimating);
+    if (isAnimating)
+      return;
     value = math.min(0.0, math.max(value + event.dx, -_kWidth));
   }
 
   void handlePointerUp(_) {
     if (!isAnimating)
       _settle();
   }
 
   void handlePointerCancel(_) {
     if (!isAnimating)
@@ skipping 113 matching lines @@
       level: level
     );
 
     return new Container(
       style: _style,
       inlineStyle: inlineStyle,
       children: [ mask, content ]
     );
   }
 }