Fork some more files from the old framework, so that the transition will be easier to perform.

sky/sdk/lib/animation/scroll_behavior.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;
+
 import 'mechanics.dart';
 import 'generators.dart';
 
 const double _kScrollFriction = 0.005;
 const double _kOverscrollFriction = 0.075;
 const double _kBounceSlopeAngle = math.PI / 512.0; // radians
 
 abstract class ScrollBehavior {
   Simulation release(Particle particle) => null;
 
@@ skipping 89 matching lines @@
     // reduce the portion of scrollDelta that's applied beyond 0.0. We
     // do similar things for overscroll in the other direction.
     if (newScrollOffset < 0.0) {
       newScrollOffset -= (newScrollOffset - math.min(0.0, scrollOffset)) / 2.0;
     } else if (newScrollOffset > maxScroll) {
       newScrollOffset -= (newScrollOffset - math.max(maxScroll, scrollOffset)) / 2.0;
     }
     return newScrollOffset;
   }
 }