Make FixedHeightScrollable understand heights

sky/sdk/lib/framework/rendering/box.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:math' as math;
 import 'dart:sky' as sky;
 import 'dart:typed_data';
 import 'object.dart';
 import 'package:vector_math/vector_math.dart';
 import 'package:sky/framework/net/image_cache.dart' as image_cache;
@@ skipping 542 matching lines @@
   }
 
   void paint(RenderObjectDisplayList canvas) {
     canvas.save();
     canvas.concat(_transform.storage);
     super.paint(canvas);
     canvas.restore();
   }
 }
 
+typedef void SizeChangedCallback(sky.Size newSize);
+
+class RenderSizeObserver extends RenderProxyBox {
+  RenderSizeObserver({
+    this.callback,
+    RenderBox child
+  }) : super(child) {
+    assert(callback != null);
+  }
+
+  SizeChangedCallback callback;
+
+  void performLayout() {
+    sky.Size oldSize = size;
+
+    super.performLayout();
+
+    if (oldSize != size)
+      callback(size);
+  }
+}
+
 
 // RENDER VIEW LAYOUT MANAGER
 
 class ViewConstraints {
 
   const ViewConstraints({
     this.width: 0.0, this.height: 0.0, this.orientation: null
   });
 
   final double width;
@@ skipping 88 matching lines @@
 
   void defaultPaint(RenderObjectDisplayList canvas) {
     RenderBox child = firstChild;
     while (child != null) {
       assert(child.parentData is ParentDataType);
       canvas.paintChild(child, child.parentData.position);
       child = child.parentData.nextSibling;
     }
   }
 }