Drive overscroll animations with a physics simulation

sky/framework/animation/scroll_curve.dart

Index: sky/framework/animation/scroll_curve.dart
diff --git a/sky/framework/animation/scroll_curve.dart b/sky/framework/animation/scroll_curve.dart
index 54b6385bd1e5d10cccb8fc4dc209ca919b0ab0d9..4aa9483297eb39c971eb988cddf105b2227d2a50 100644
--- a/sky/framework/animation/scroll_curve.dart
+++ b/sky/framework/animation/scroll_curve.dart
@@ -3,8 +3,14 @@
 // found in the LICENSE file.
 
 import 'dart:math' as math;
+import 'kinematics.dart';
+import 'simulation.dart';
+
+const double _kSlope = 0.01;
 
 abstract class ScrollCurve {
+  Simulation release(Particle particle) => null;
+
   // Returns the new scroll offset.
   double apply(double scrollOffset, double scrollDelta);
 }
@@ -26,6 +32,18 @@ class BoundedScrollCurve extends ScrollCurve {
 }
 
 class OverscrollCurve extends ScrollCurve {
+  Simulation release(Particle particle) {
+    if (particle.position >= 0.0)
+      return null;
+    System system = new ParticleClimbingHill(
+        particle: particle,
+        box: new Box(max: 0.0),
+        slope: _kSlope,
+        targetPosition: 0.0);
+    return new Simulation(system,
+        terminationCondition: () => particle.position == 0.0);
+  }
+
   double apply(double scrollOffset, double scrollDelta) {
     double newScrollOffset = scrollOffset + scrollDelta;
     if (newScrollOffset < 0.0) {