Refactor fn2.dart, since it breached our 1000-line threshold.

sky/sdk/lib/framework/components2/fixed_height_scrollable.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 '../animation/scroll_behavior.dart';
-import '../fn2.dart';
-import 'dart:async';
-import 'dart:math' as math;
-import 'package:vector_math/vector_math.dart';
-import 'scrollable.dart';
-
-abstract class FixedHeightScrollable extends Scrollable {
-
-  FixedHeightScrollable({ this.itemHeight, Object key }) : super(key: key) {
-    assert(itemHeight != null);
-  }
-
-  double itemHeight;
-
-  void syncFields(FixedHeightScrollable source) {
-    itemHeight = source.itemHeight;
-    super.syncFields(source);
-  }
-
-  ScrollBehavior createScrollBehavior() => new OverscrollBehavior();
-  OverscrollBehavior get scrollBehavior => super.scrollBehavior as OverscrollBehavior;
-
-  int _itemCount = 0;
-  int get itemCount => _itemCount;
-  void set itemCount (int value) {
-    if (_itemCount != value) {
-      _itemCount = value;
-      scrollBehavior.contentsHeight = itemHeight * _itemCount;
-    }
-  }
-
-  double _height;
-  void _handleSizeChanged(Size newSize) {
-    setState(() {
-      _height = newSize.height;
-      scrollBehavior.containerHeight = _height;
-    });
-  }
-
-  UINode buildContent() {
-    var itemShowIndex = 0;
-    var itemShowCount = 0;
-
-    Matrix4 transform = new Matrix4.identity();
-
-    if (_height != null && _height > 0.0) {
-      if (scrollOffset < 0.0) {
-        double visibleHeight = _height + scrollOffset;
-        itemShowCount = (visibleHeight / itemHeight).round() + 1;
-        transform.translate(0.0, -scrollOffset);
-      } else {
-        itemShowCount = (_height / itemHeight).ceil() + 1;
-        double alignmentDelta = -scrollOffset % itemHeight;
-        if (alignmentDelta != 0.0)
-          alignmentDelta -= itemHeight;
-
-        double drawStart = scrollOffset + alignmentDelta;
-        itemShowIndex = math.max(0, (drawStart / itemHeight).floor());
-
-        transform.translate(0.0, alignmentDelta);
-      }
-    }
-
-    return new SizeObserver(
-      callback: _handleSizeChanged,
-      child: new ClipRect(
-        child: new DecoratedBox(
-          decoration: const BoxDecoration(
-            backgroundColor: const Color(0xFFFFFFFF)
-          ),
-          child: new Transform(
-            transform: transform,
-            child: new Block(buildItems(itemShowIndex, itemShowCount))
-          )
-        )
-      )
-    );
-  }
-
-  List<UINode> buildItems(int start, int count);  
-
-}