remove unnecessary @required (#10501)

packages/flutter/lib/src/cupertino/page.dart

@@ -114,7 +114,7 @@ class _CupertinoEdgeShadowDecoration extends Decoration {
 /// A [BoxPainter] used to draw the page transition shadow using gradients.
 class _CupertinoEdgeShadowPainter extends BoxPainter {
   _CupertinoEdgeShadowPainter(
-    @required this._decoration,
+    this._decoration,
     VoidCallback onChange
   ) : assert(_decoration != null),
       super(onChange);

packages/flutter/lib/src/painting/box_painter.dart

@@ -1659,7 +1659,7 @@ class BoxDecoration extends Decoration {
 
 /// An object that paints a [BoxDecoration] into a canvas.
 class _BoxDecorationPainter extends BoxPainter {
-  _BoxDecorationPainter(@required this._decoration, VoidCallback onChange)
+  _BoxDecorationPainter(this._decoration, VoidCallback onChange)
     : assert(_decoration != null),
       super(onChange);