[test] Fix expected type in error message

We missed the nullable type in the expected strings during the
migration.

Change-Id: Ie354e89a0ba0ca1a74caaffcc41c72655c280f04
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/153704
Auto-Submit: Nicholas Shahan <[email protected]>
Reviewed-by: Bob Nystrom <[email protected]>
Commit-Queue: Bob Nystrom <[email protected]>

Author: nshahan

tests/language/unsorted/checked_method_error_order_test.dart

@@ -35,9 +35,9 @@ main() {
     Expect.throws(() => new Bar().optional(i: x, a: y), (e) {
       if (e is TypeError) {
         var m = e.toString();
-        return m.contains("is not a subtype of type 'int'") ||
+        return m.contains("is not a subtype of type 'int?'") ||
             m.contains(
-                "Expected a value of type 'int', but got one of type 'String'");
+                "Expected a value of type 'int?', but got one of type 'String'");
       }
       return false;
     });