fix some errors

com.unity.uiwidgets/Runtime/cupertino/dialog.cs

                     textScaleFactor: Mathf.Max(textScaleFactor, 1.0f)
                 ),
                 child: new LayoutBuilder(
-                    builder: (BuildContext _context, BoxConstraints constraints) => {
+                    builder: (BuildContext _context, Constraints constraints) => {
                         return new Center(
                             child: new Container(
                                 margin: EdgeInsets.symmetric(vertical: CupertinoDialogUtils._kEdgePadding),

com.unity.uiwidgets/Runtime/widgets/orientation_builder.cs

 
         public readonly OrientationWidgetBuilder builder;
 
-        Widget _buildWithConstraints(BuildContext context, BoxConstraints constraints) {
+        Widget _buildWithConstraints(BuildContext context, Constraints constraints) {
-                constraints.maxWidth > constraints.maxHeight ? Orientation.landscape : Orientation.portrait;
+                ((BoxConstraints)constraints).maxWidth > ((BoxConstraints)constraints).maxHeight ? Orientation.landscape : Orientation.portrait;
             return builder(context, orientation);
         }
 

com.unity.uiwidgets/Runtime/widgets/page_view.cs

                             slivers: new List<Widget> {
                                 new SliverFillViewport(
                                     viewportFraction: widget.controller.viewportFraction,
-                                    del: widget.childrenDelegate
+                                    _delegate: widget.childrenDelegate
                                 )
                             }
                         );