Fix logic in RenderPadding.

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 'node.dart';
 import 'dart:sky' as sky;
+import 'dart:math' as math;
 
 // GENERIC BOX RENDERING
 // Anything that has a concept of x, y, width, height is going to derive from this
 
 class EdgeDims {
   // used for e.g. padding
   const EdgeDims(this.top, this.right, this.bottom, this.left);
   const EdgeDims.all(double value)
       : top = value, right = value, bottom = value, left = value;
 
@@ skipping 16 matching lines @@
     this.maxHeight: double.INFINITY});
 
   BoxConstraints.tight(sky.Size size)
     : minWidth = size.width,
       maxWidth = size.width,
       minHeight = size.height,
       maxHeight = size.height;
 
   BoxConstraints deflate(EdgeDims edges) {
     assert(edges != null);
+    var horizontal = edges.left + edges.right;
+    var vertical = edges.top + edges.bottom;

[abarth-chromium, 2015/06/03 17:22:33]

s/var/double/

     return new BoxConstraints(
-      minWidth: minWidth,
-      maxWidth: maxWidth - (edges.left + edges.right),
-      minHeight: minHeight,
-      maxHeight: maxHeight - (edges.top + edges.bottom)
+      minWidth: math.min(0.0, minWidth - horizontal),
+      maxWidth: maxWidth - horizontal,
+      minHeight: math.min(0.0, minHeight - vertical),
+      maxHeight: maxHeight - vertical
     );
   }
 
   final double minWidth;
   final double maxWidth;
   final double minHeight;
   final double maxHeight;
 
   double constrainWidth(double width) {
     return clamp(min: minWidth, max: maxWidth, value: width);
@@ skipping 106 matching lines @@
     _desiredSize = value;
     markNeedsLayout();
   }
 
   sky.Size getIntrinsicDimensions(BoxConstraints constraints) {
     return constraints.constrain(_desiredSize);
   }
 
   void performLayout() {
     size = constraints.constrain(_desiredSize);
-    child.layout(new BoxConstraints.tight(size));
+    if (child != null)
+      child.layout(new BoxConstraints.tight(size));
   }
 }
 
 class RenderPadding extends RenderBox with RenderNodeWithChildMixin<RenderBox> {
 
   RenderPadding({ EdgeDims padding, RenderBox child }) {
     assert(padding != null);
     this.padding = padding;
     this.child = child;
   }
@@ skipping 193 matching lines @@
 
   void defaultPaint(RenderNodeDisplayList canvas) {
     RenderBox child = firstChild;
     while (child != null) {
       assert(child.parentData is ParentDataType);
       canvas.paintChild(child, child.parentData.position);
       child = child.parentData.nextSibling;
     }
   }
 }