Improve Sky's Scrollable physics

sky/framework/animation/mechanics.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 'dart:math' as math;
 
 const double kGravity = -0.980;
-const double _kMinVelocity = 0.1;
+const double _kMinVelocity = 0.01;
 
 abstract class System {
   void update(double deltaT);
 }
 
 class Particle extends System {
   final double mass;
   double velocity;
   double position;
 
@@ skipping 43 matching lines @@
   ParticleInBox({this.particle, this.box}) {
     box.confine(particle);
   }
 
   void update(double deltaT) {
     particle.update(deltaT);
     box.confine(particle);
   }
 }
 
+class ParticleInBoxWithFriction extends ParticleInBox {
+  final double friction;
+  final double _sign;
+
+  ParticleInBoxWithFriction({Particle particle, Box box, this.friction})
+      : super(particle: particle, box: box),
+        _sign = particle.velocity.sign;
+
+  void update(double deltaT) {
+    double force = -_sign * friction;
+    particle.applyImpluse(force * deltaT);
+    if (particle.velocity.sign != _sign)
+      particle.velocity = 0.0;
+    super.update(deltaT);
+  }
+}
+
 class Spring {
   final double k;
   double displacement;
 
   Spring(this.k, {this.displacement: 0.0});
 
   double get force => -k * displacement;
 }
 
 class ParticleAndSpringInBox extends System {
@@ skipping 35 matching lines @@
   void update(double deltaT) {
     particle.applyImpluse(kGravity * slope * deltaT);
     // If we don't apply a min velocity, error terms in the simulation can
     // prevent us from reaching the targetPosition before gravity overtakes our
     // initial velocity and we start rolling down the hill.
     particle.velocity = math.max(_kMinVelocity, particle.velocity);
     particle.update(deltaT);
     box.confine(particle);
   }
 }