Merge pull request #514 from apwilson/image_assert

Fix asserts related to image drawing.

packages/flutter/lib/rendering/box.dart

@@ -1406,7 +1406,7 @@ class RenderImage extends RenderBox {
         return constraints.constrain(new Size(width, height));
       }
     }
-    return constraints.constrain(new Size(width, height));
+    return constraints.constrain(new Size(_image.width.toDouble(), _image.height.toDouble()));
   }
 
   double getMinIntrinsicWidth(BoxConstraints constraints) {