[Cupertino] Fix some accidental changes in samples.

Author: Yuncong Zhang

Samples/UIWidgetsGallery/demo/animation/home.cs

@@ -340,7 +340,7 @@ float _indicatorOpacity(int index) {
             );
         }
 
-        protected internal override Widget build(BuildContext context) {
+        protected override Widget build(BuildContext context) {
             return new LayoutBuilder(builder: this._build);
         }
     }

Samples/UIWidgetsGallery/demo/material/backdrop_demo.cs

@@ -222,7 +222,7 @@ public BackdropTitle(
         ) : base(key: key, listenable: listenable) {
         }
 
-        protected internal override Widget build(BuildContext context) {
+        protected override Widget build(BuildContext context) {
             Animation<float> animation = (Animation<float>) this.listenable;
             return new DefaultTextStyle(
                 style: Theme.of(context).primaryTextTheme.title,

Samples/UIWidgetsGallery/gallery/backdrop.cs

@@ -95,7 +95,7 @@ public _CrossFadeTransition(
         public readonly Widget child0;
         public readonly Widget child1;
 
-        protected internal override Widget build(BuildContext context) {
+        protected override Widget build(BuildContext context) {
             Animation<float> progress = this.listenable as Animation<float>;
 
             float opacity1 = new CurvedAnimation(