spring can be null drag must not be

packages/newton/lib/src/scroll_simulation.dart

@@ -18,12 +18,11 @@ class ScrollSimulation extends SimulationGroup {
   double _offset = 0.0;
 
   ScrollSimulation(double position, double velocity, double leading,
-      double trailing, [SpringDescription spring, double drag])
+      double trailing, SpringDescription spring, double drag)
       : _leadingExtent = leading,
         _trailingExtent = trailing,
         _springDesc = spring,
         _drag = drag {
-    assert((spring != null && drag != null) || (spring == null && drag == null));
     _chooseSimulation(position, velocity, 0.0);
   }