Fix an issue.

Runtime/widgets/framework.cs

     }
 
     public abstract class StatefulWidget : Widget {
-        protected StatefulWidget(Key key = null) : base(key: key) {
+        protected StatefulWidget(Key key) : base(key: key) {
         }
 
         public override Element createElement() {

Samples/UIWidgetsGallery/gallery/backdrop.cs

             Widget frontLayer = null,
             Widget backTitle = null,
             Widget backLayer = null
-        ) {
+        ) : base(key: null) {
             this.frontAction = frontAction;
             this.frontTitle = frontTitle;
             this.frontHeading = frontHeading;