add public to layoutBuilder

Runtime/widgets/layout_builder.cs

 namespace Unity.UIWidgets.widgets {
     public delegate Widget LayoutWidgetBuilder(BuildContext context, BoxConstraints constraints);
 
-
-    class LayoutBuilder : RenderObjectWidget {
+    public class LayoutBuilder : RenderObjectWidget {
         public LayoutBuilder(
             Key key = null,
             LayoutWidgetBuilder builder = null) : base(key: key) {