Fix mistakes in variance_downwards_inference_test.

Fix flutter#43422.

Change-Id: Ic315148284e45a26cfe88ac6177c16c671a318eb
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/162961
Commit-Queue: Bob Nystrom <[email protected]>
Reviewed-by: Erik Ernst <[email protected]>

Author: munificent

tests/language/variance/variance_downwards_inference_test.dart

@@ -10,7 +10,7 @@ class A<out T> {
   final T _x;
   A(T x):_x = x;
   T get x => _x;
-  void set x(Object value) {}
+  void set x(Object? value) {}
 }
 
 class B<in T> {
@@ -22,7 +22,7 @@ class C<out T, S> {
   final T _x;
   C(T x, S y):_x = x;
   T get x => _x;
-  void set x(Object value) {}
+  void set x(Object? value) {}
   void set y(S _value) {}
 }