Remove the "min velocity" hack from ParticleClimbingRamp

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 = 0.01;
 
 abstract class System {
   void update(double deltaT);
 }
 
 class Particle extends System {
   final double mass;
   double velocity;
   double position;
 
@@ skipping 106 matching lines @@
       this.particle,
       this.box,
       this.slope,
       double targetPosition}) {
     double deltaPosition = targetPosition - particle.position;
     particle.energy = -kGravity * slope * deltaPosition * particle.mass;
     box.confine(particle);
   }
 
   void update(double deltaT) {
+    particle.update(deltaT);
+    // Note that we apply the impluse from gravity after updating the particle's
+    // position so that we overestimate the distance traveled by the particle.
+    // That ensures that we actually hit the edge of the box and don't wind up
+    // reversing course.
     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);
   }
 }