Remove duplicate copy of fling-curve.dart in fn

sky/examples/fn/widgets/flingcurve.dart

-part of widgets;
-
-// 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.
-
-const double _kDefaultAlpha = -5707.62;
-const double _kDefaultBeta = 172.0;
-const double _kDefaultGamma = 3.7;
-
-double _positionAtTime(double t) {
-  return _kDefaultAlpha * math.exp(-_kDefaultGamma * t)
-      - _kDefaultBeta * t
-      - _kDefaultAlpha;
-}
-
-double _velocityAtTime(double t) {
-  return -_kDefaultAlpha * _kDefaultGamma * math.exp(-_kDefaultGamma * t)
-      - _kDefaultBeta;
-}
-
-double _timeAtVelocity(double v) {
-  return -math.log((v + _kDefaultBeta) / (-_kDefaultAlpha * _kDefaultGamma))
-      / _kDefaultGamma;
-}
-
-final double _kMaxVelocity = _velocityAtTime(0.0);
-final double _kCurveDuration = _timeAtVelocity(0.0);
-
-class FlingCurve {
-  double _timeOffset;
-  double _positionOffset;
-  double _startTime;
-  double _previousPosition;
-  double _direction;
-
-  FlingCurve(double velocity, double startTime) {
-    double startingVelocity = math.min(_kMaxVelocity, velocity.abs());
-    _timeOffset = _timeAtVelocity(startingVelocity);
-    _positionOffset = _positionAtTime(_timeOffset);
-    _startTime = startTime / 1000.0;
-    _previousPosition = 0.0;
-    _direction = velocity.sign;
-  }
-
-  double update(double timeStamp) {
-    double t = timeStamp / 1000.0 - _startTime + _timeOffset;
-    if (t >= _kCurveDuration)
-      return 0.0;
-    double position = _positionAtTime(t) - _positionOffset;
-    double positionDelta = position - _previousPosition;
-    _previousPosition = position;
-    return _direction * math.max(0.0, positionDelta);
-  }
-}