Port sky-scrollable to Dart

sky/framework/fling-curve.sky

-<!--
-// 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.
--->
-<script>
-const kDefaultAlpha = -5707.62;
-const kDefaultBeta = 172;
-const kDefaultGamma = 3.7;
-
-function positionAtTime(t) {
-  return kDefaultAlpha * Math.exp(-kDefaultGamma * t) - kDefaultBeta * t - kDefaultAlpha;
-}
-
-function velocityAtTime(t) {
-  return -kDefaultAlpha * kDefaultGamma * Math.exp(-kDefaultGamma * t) - kDefaultBeta;
-}
-
-function timeAtVelocity(v) {
-  return -Math.log((v + kDefaultBeta) / (-kDefaultAlpha * kDefaultGamma)) / kDefaultGamma;
-}
-
-var kMaxVelocity = velocityAtTime(0);
-var kCurveDuration = timeAtVelocity(0);
-
-module.exports = class FlingCurve {
-  constructor(velocity, startTime) {
-    var startingVelocity = Math.min(kMaxVelocity, Math.abs(velocity));
-    this.timeOffset_ = timeAtVelocity(startingVelocity);
-    this.positionOffset_ = positionAtTime(this.timeOffset_);
-    this.startTime_ = startTime / 1000;
-    this.previousPosition_ = 0;
-    this.direction_ = Math.sign(velocity);
-    Object.preventExtensions(this);
-  }
-
-  update(timeStamp) {
-    var t = timeStamp / 1000 - this.startTime_ + this.timeOffset_;
-    if (t >= kCurveDuration)
-      return 0;
-    var position = positionAtTime(t) - this.positionOffset_;
-    var positionDelta = position - this.previousPosition_;
-    this.previousPosition_ = position;
-    return this.direction_ * Math.max(0, positionDelta);
-  }
-};
-</script>