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

sky/framework/components/fixed_height_scrollable.dart

Index: sky/framework/components/fixed_height_scrollable.dart
diff --git a/sky/framework/components/fixed_height_scrollable.dart b/sky/framework/components/fixed_height_scrollable.dart
index 3f3e973d8b3e3d36c1f46ed81ce5b515aaf037cb..2838acbfa6a230bf32d0549c7c123b92ac17c023 100644
--- a/sky/framework/components/fixed_height_scrollable.dart
+++ b/sky/framework/components/fixed_height_scrollable.dart
@@ -6,7 +6,6 @@ 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';
 
@@ -52,14 +51,11 @@ abstract class FixedHeightScrollable extends Scrollable {
       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;
       });
@@ -82,7 +78,6 @@ abstract class FixedHeightScrollable extends Scrollable {
           '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;