RenderStack should handle top, right, bottom, left all being zero

Previously, we didn't set the width of the child correctly when top and bottom
(or left and right) were both specified.

Fixes #275

packages/flutter/lib/rendering/stack.dart

@@ -155,14 +155,14 @@ class RenderStack extends RenderBox with ContainerRenderObjectMixin<RenderBox, S
         BoxConstraints childConstraints = innerConstraints;
 
         if (childData.left != null && childData.right != null)
-          childConstraints = childConstraints.applyWidth(childData.right - childData.left);
+          childConstraints = childConstraints.applyWidth(size.width - childData.right - childData.left);
         else if (childData.left != null)
           childConstraints = childConstraints.applyMaxWidth(size.width - childData.left);
         else if (childData.right != null)
           childConstraints = childConstraints.applyMaxWidth(size.width - childData.right);
 
         if (childData.top != null && childData.bottom != null)
-          childConstraints = childConstraints.applyHeight(childData.bottom - childData.top);
+          childConstraints = childConstraints.applyHeight(size.height - childData.bottom - childData.top);
         else if (childData.top != null)
           childConstraints = childConstraints.applyMaxHeight(size.height - childData.top);
         else if (childData.bottom != null)