Two minor improvements (#4554)

Add `@required` for some required parameters in `RRect`. Use elvis
operator to simplify a function.

packages/flutter/lib/src/rendering/layer.dart

@@ -33,8 +33,7 @@ abstract class Layer {
   /// Removes this layer from its parent layer's child list
   @mustCallSuper
   void detach() {
-    if (_parent != null)
-      _parent._remove(this);
+    _parent?._remove(this);
   }
 
   /// Replaces this layer with the given layer in the parent layer's child list

packages/flutter/lib/src/widgets/basic.dart

@@ -310,8 +310,12 @@ class ClipRect extends SingleChildRenderObjectWidget {
 /// rectangle.
 class ClipRRect extends SingleChildRenderObjectWidget {
   /// Creates a rounded-rectangular clip.
-  ClipRRect({ Key key, this.xRadius, this.yRadius, Widget child })
-    : super(key: key, child: child);
+  ClipRRect({
+    Key key,
+    @required this.xRadius,
+    @required this.yRadius,
+    Widget child
+  }) : super(key: key, child: child);
 
   /// The radius of the rounded corners in the horizontal direction in logical pixels.
   ///