fix the hang when clicking the drawer in the stocks app

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/980043005

examples/fn/widgets/drawer.dart

@@ -73,10 +73,12 @@ class DrawerAnimation {
 
   void _animateToPosition(double targetPosition) {
     double distance = (targetPosition - _position).abs();
+    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;