Set default borderRadius to zero in ClipRRect (as documented) (#43006)

* Set default borderRadius to none in ClipRRect

* add tests

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

@@ -647,7 +647,7 @@ class ClipRRect extends SingleChildRenderObjectWidget {
   /// The [clipBehavior] argument must not be null or [Clip.none].
   const ClipRRect({
     Key key,
-    this.borderRadius,
+    this.borderRadius = BorderRadius.zero,
     this.clipper,
     this.clipBehavior = Clip.antiAlias,
     Widget child,

packages/flutter/test/widgets/clip_test.dart

@@ -92,6 +92,12 @@ void main() {
     expect(renderClip.clipBehavior, equals(Clip.hardEdge));
   });
 
+  test('ClipRRect constructs with the right default values', () {
+    const ClipRRect clipRRect = ClipRRect();
+    expect(clipRRect.clipBehavior, equals(Clip.antiAlias));
+    expect(clipRRect.borderRadius, equals(BorderRadius.zero));
+  });
+
   testWidgets('ClipRRect updates clipBehavior in updateRenderObject', (WidgetTester tester) async {
     await tester.pumpWidget(_UpdateCountedClipRRect());