[Effen] Port fn.dart from the legacy sky.Node backend to the RenderNode backend, which is currently…

sky/framework/components/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 '../debug/tracing.dart';
 import '../fn.dart';
 import 'dart:math' as math;
-import 'dart:sky' as sky;
 import 'dart:async';
 import 'scrollable.dart';
 
 abstract class FixedHeightScrollable extends Scrollable {
   static final Style _style = new Style('''
     overflow: hidden;
     position: relative;
     will-change: transform;'''
   );
 
@@ skipping 25 matching lines @@
   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;
-      sky.ClientRect scrollRect = root.getBoundingClientRect();
-      sky.ClientRect itemRect = item.getBoundingClientRect();
-      assert(scrollRect.height > 0);
-      assert(itemRect.height > 0);
-
       setState(() {
-        _height = scrollRect.height;
-        _itemHeight = itemRect.height;
+        _height = root.height;
+        assert(_height > 0);
+        _itemHeight = item.height;
+        assert(_itemHeight > 0);
         scrollBehavior.containerHeight = _height;
         scrollBehavior.contentsHeight = _itemHeight * _itemCount;
       });
     });
   }
 
   UINode buildContent() {
     var itemNumber = 0;
     var drawCount = 1;
     var transformStyle = '';
 
     if (_itemHeight == null)
       new Future.microtask(_measureHeights);
 
     if (_height > 0.0 && _itemHeight != null) {
       if (scrollOffset < 0.0) {
         double visibleHeight = _height + scrollOffset;
         drawCount = (visibleHeight / _itemHeight).round() + 1;
         transformStyle =
           'transform: translateY(${(-scrollOffset).toStringAsFixed(2)}px)';
       } else {
         drawCount = (_height / _itemHeight).round() + 1;
-        double alignmentOffset = math.max(0.0, scrollOffset);
         double alignmentDelta = -scrollOffset % _itemHeight;
         if (alignmentDelta != 0.0)
           alignmentDelta -= _itemHeight;
 
         double drawStart = scrollOffset + alignmentDelta;
         itemNumber = math.max(0, (drawStart / _itemHeight).floor());
 
         transformStyle =
             'transform: translateY(${(alignmentDelta).toStringAsFixed(2)}px)';
       }
     }
 
     return new Container(
       style: _style,
       children: [
         new Container(
           style: _scrollAreaStyle,
           inlineStyle: transformStyle,
           children: buildItems(itemNumber, drawCount)
         )
       ]
     );
   }
 
   List<UINode> buildItems(int start, int count);  
 }