Fix nnbd/syntax/required_modifier_error_test

There were compilation errors reported because of missing default
values for non-required optional non-nullable parameters.

[email protected], [email protected]

Change-Id: If0454988219a7fece317c86124a81a2a8ce8bea4
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/106963
Reviewed-by: Brian Wilkerson <[email protected]>
Reviewed-by: Dan Rubel <[email protected]>
Commit-Queue: Konstantin Shcheglov <[email protected]>

Author: scheglov

tests/language_2/nnbd/syntax/required_modifier_error_test.dart

@@ -29,12 +29,9 @@ void f2({
 // Out of order modifiers
 class C2 {
   void m({
-    covariant
-    required //# 07: compile-time error
-    int i2,
-    final
-    required //# 08: compile-time error
-    int i3,
+    required int i1,
+    covariant required int i2, //# 07: compile-time error
+    final required int i3, //# 08: compile-time error
   }) {
   }
 }