Fix subtle bugs in sky-scrollable

sky/framework/fling-curve.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 _kDefaultAlpha = -5707.62;
 const double _kDefaultBeta = 172.0;
 const double _kDefaultGamma = 3.7;
 
@@ skipping 18 matching lines @@
 
 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 = _velocityAtTime(startingVelocity);
+    _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);
   }
 }