minor fixes

preferredSize widget is made to be a statefulwidget. it is not the same as in flutter, but works the same. you can refer to the comments for the reason

com.unity.uiwidgets/Runtime/material/scaffold.cs

 
         public readonly Completer _completer;
 
-        public Completer closed {
-            get { return _completer; }
+        public Future closed {
+            get { return _completer.future; }
         }
 
         public readonly VoidCallback close;

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

         Size preferredSize { get; }
     }
 
-    public abstract class PreferredSizeWidget : StatefulWidget{
+    public abstract class PreferredSizeWidget : StatefulWidget, SizePreferred {
         protected PreferredSizeWidget(Key key = null) : base(key: key) {
         }
 
-
-    public class PreferredSize : StatelessWidget{
+    /**
+     *  In flutter this class inherits StatelessWidget explicitly, in particular, the class inheritance diagram looks like follows:
+     *  PreferredSizeWidget   <--   Widget
+     *           |                    |
+     *          v                    v
+     *  PreferredSize       <--    StatelessWidget
+     * 
+     *  It is hard to implement such a complex inheritance diagram in C#. so we choose to create a slightly different diagram, which is more simple and making every C# code happy:
+     *
+     *  PreferredSize      <--    PreferredSizeWidget    <--    StatefulWidget   <--    Widget
+     *
+     *  The bad side is that, we should make all classes that inherit PreferredSizeWidget to be statefulWidget now. Though it is not that bad since a statelessWidget can be made a statefulWidget easily
+     * 
+     */
+    public class PreferredSize : PreferredSizeWidget {
         public PreferredSize(
             Key key = null,
             Widget child = null,
         }
 
         public readonly Widget child;
-
-        //public override Size preferredSize { get; }
-        public Size preferredSize { get; }
-        /*public override State createState() {
+        public override Size preferredSize { get; }
+        
+        public override State createState() {
-        }*/
-        public override Widget build(BuildContext context) {
-            return child;
-            //throw new System.NotImplementedException();
-    /*class _PreferredSizeState : State<PreferredSize> {
-        public override Widget build(BuildContext context) {
+    class _PreferredSizeState : State<PreferredSize> {
+        public override Widget build(BuildContext buildContext) {
-    }*/
+    }
 }