Move VariableHeightScrollable to sdk/lib/widgets

sky/sdk/lib/widgets/block_viewport.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 'dart:collection';
+
 import '../rendering/block.dart';
 import '../rendering/box.dart';
 import '../rendering/object.dart';
 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);
   final Type type;
   final String key;
   bool operator ==(other) => other is _Key && other.type == type && other.key == key;
   int get hashCode => 373 * 37 * type.hashCode + key.hashCode;
 }
 
 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();
 
   Map<_Key, Widget> _childrenByKey = new Map<_Key, Widget>();  
 
   void walkChildren(WidgetTreeWalker walker) {
     for (Widget child in _childrenByKey.values)
       walker(child);
   }
@@ skipping 241 matching lines @@
           root.add(widget.root, before: nextSibling);
         }
         widget.updateSlot(nextSibling);
         nextSibling = widget.root;
       }      
     }
 
     _childrenByKey = newChildren;
     _currentStartIndex = startIndex;
     _currentChildCount = _childrenByKey.length;
+
+    if (onLayoutChanged != null)
+      onLayoutChanged(_currentStartIndex, _currentChildCount, new UnmodifiableListView<double>(_offsets));
   }
 
 }