fix the hang when clicking the drawer in the stocks app

sky/examples/fn/widgets/drawer.dart

 part of widgets;
 
 const double _kWidth = 256.0;
 const double _kMinFlingVelocity = 0.4;
 const double _kBaseSettleDurationMS = 246.0;
 const double _kMaxSettleDurationMS = 600.0;
 const Cubic _kAnimationCurve = easeOut;
 
 class DrawerAnimation {
 
@@ skipping 55 matching lines @@
     _animation = new AnimationGenerator(duration, begin: begin, end: end,
         curve: curve);
 
     _animation.onTick.listen(_setPosition, onDone: () {
       _animation = null;
     });
   }
 
   void _animateToPosition(double targetPosition) {
     double distance = (targetPosition - _position).abs();
-    double targetDuration = distance / _kWidth * _kBaseSettleDurationMS;
-    double duration = math.min(targetDuration, _kMaxSettleDurationMS);
-    _animate(duration, _position, targetPosition, _kAnimationCurve);
+    if (distance != 0) {
+      double targetDuration = distance / _kWidth * _kBaseSettleDurationMS;
+      double duration = math.min(targetDuration, _kMaxSettleDurationMS);
+      _animate(duration, _position, targetPosition, _kAnimationCurve);
+    }
   }
 
   void handleFlingStart(event) {
     double direction = event.velocityX.sign;
     double velocityX = event.velocityX.abs() / 1000;
     if (velocityX < _kMinFlingVelocity)
       return;
 
     double targetPosition = direction < 0.0 ? -_kWidth : 0.0;
     double distance = (targetPosition - _position).abs();
@@ skipping 87 matching lines @@
       style: _style,
       inlineStyle: inlineStyle,
       children: [ mask, content ]
     )..events.listen('pointerdown', animation.handlePointerDown)
      ..events.listen('pointermove', animation.handlePointerMove)
      ..events.listen('pointerup', animation.handlePointerUp)
      ..events.listen('pointercancel', animation.handlePointerCancel);
 
   }
 }