anchor.diff:  // found in the LICENSE file.
   
   import '../animation/scroll_behavior.dart';
  -import '../debug/tracing.dart';
   import '../fn2.dart';
  +import 'dart:async';
   import 'dart:math' as math;
  -import 'dart:async';
  +import 'package:vector_math/vector_math.dart';
   import 'scrollable.dart';
   
   abstract class FixedHeightScrollable extends Scrollable {
  -  static final Style _style = new Style('''
  -    overflow: hidden;
  -    position: relative;
  -    will-change: transform;'''
  -  );
  -
  -  static final Style _scrollAreaStyle = new Style('''
  -    position:relative;
  -    will-change: transform;'''
  -  );
  -
  -  FixedHeightScrollable({
  -    Object key
  -  }) : super(key: key);
  +  FixedHeightScrollable({ Object key }) : super(key: key);
   
     ScrollBehavior createScrollBehavior() => new OverscrollBehavior();
     OverscrollBehavior get scrollBehavior => super.scrollBehavior as OverscrollBehavior;
  
positive.code:   }
  
    void _measureHeights() {
      trace('FixedHeightScrollable::_measureHeights', () {
        if (_itemHeight != null)
          return;
        var root = getRoot();
        if (root == null)
          return;
        var item = root.firstChild.firstChild;
        if (item == null)
          return;
        setState(() {
          _height = root.height;
          assert(_height > 0);
          _itemHeight = item.height;
          assert(_itemHeight > 0);
          scrollBehavior.containerHeight = _height;
          scrollBehavior.contentsHeight = _itemHeight * _itemCount;
        });
      });
    }
  
  
negative.code:     if (_itemHeight == null)
        new Future.microtask(_measureHeights);
  
      Matrix4 transform = new Matrix4.identity();
  
      if (_height > 0.0 && _itemHeight != null) {
        if (scrollOffset < 0.0) {
          double visibleHeight = _height + scrollOffset;
          drawCount = (visibleHeight / _itemHeight).round() + 1;
          transform.translate(0.0, -scrollOffset);
        } else {
          drawCount = (_height / _itemHeight).ceil() + 1;
          double alignmentDelta = -scrollOffset % _itemHeight;
  