Fix Sliver.

Runtime/widgets/sliver.cs

         }
     }
 
-    class SliverFillRemaining : SingleChildRenderObjectWidget {
+    public class SliverFillRemaining : SingleChildRenderObjectWidget {
         public SliverFillRemaining(
             Key key = null,
             Widget child = null
             properties.add(new DiagnosticsProperty<bool>("keepAlive", this.keepAlive));
         }
     }
-}
+}