[tests] Fix nullable test for dot shorthands.

Fixing a couple typos keeping this test from passing.

Bug: #59758
Change-Id: I5fd8345538ee4516db4565c910dcb75b93c4cda1
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/414185
Commit-Queue: Kallen Tu <[email protected]>
Reviewed-by: Bob Nystrom <[email protected]>

Author: kallentu

tests/language/dot_shorthands/simple/simple_identifier_nullable_test.dart

@@ -42,7 +42,7 @@ void main() {
   // Mixin
   IntegerMixin? integerMixin = .mixinOne;
   const IntegerMixin? constIntegerMixin = .mixinConstOne;
-  var integerMixinList = <IntegerMixin?>[.one, .two, .one];
+  var integerMixinList = <IntegerMixin?>[.mixinOne, .mixinTwo, .mixinOne];
 
   // Null assertion on a nullable static member.
   NullableInteger? nullableInteger = .one;