Move VariableHeightScrollable to sdk/lib/widgets

sky/sdk/lib/widgets/block_viewport.dart

Index: sky/sdk/lib/widgets/block_viewport.dart
diff --git a/sky/sdk/lib/widgets/block_viewport.dart b/sky/sdk/lib/widgets/block_viewport.dart
index 8aa0f0c3756dd5379739ca19977e2cf0cc7417af..c3d7ddc2417292dfbb6d5765d9ff13859f479b94 100644
--- a/sky/sdk/lib/widgets/block_viewport.dart
+++ b/sky/sdk/lib/widgets/block_viewport.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:collection';
+
 import '../rendering/block.dart';
 import '../rendering/box.dart';
 import '../rendering/object.dart';
@@ -10,6 +12,11 @@ import 'widget.dart';
 // return null if index is greater than index of last entry
 typedef Widget IndexedBuilder(int index);
 
+typedef void LayoutChangedCallback(
+  int firstVisibleItemIndex, 
+  int visibleItemCount,
+  UnmodifiableListView<double> itemOffsets);
+
 class _Key {
   const _Key(this.type, this.key);
   factory _Key.fromWidget(Widget widget) => new _Key(widget.runtimeType, widget.key);
@@ -20,12 +27,13 @@ class _Key {
 }
 
 class BlockViewport extends RenderObjectWrapper {
-  BlockViewport({ this.builder, this.startOffset, this.token, String key })
+  BlockViewport({ this.builder, this.startOffset, this.token, this.onLayoutChanged, String key })
     : super(key: key);
 
   IndexedBuilder builder;
   double startOffset;
   Object token;
+  LayoutChangedCallback onLayoutChanged;
 
   RenderBlockViewport get root => super.root;
   RenderBlockViewport createNode() => new RenderBlockViewport();
@@ -287,6 +295,9 @@ class BlockViewport extends RenderObjectWrapper {
     _childrenByKey = newChildren;
     _currentStartIndex = startIndex;
     _currentChildCount = _childrenByKey.length;
+
+    if (onLayoutChanged != null)
+      onLayoutChanged(_currentStartIndex, _currentChildCount, new UnmodifiableListView<double>(_offsets));
   }
 
 }