[test] Fix expected and actual ordering

Not a big deal but I was confused for longer than I would like to
admit because the failure message labels the "expected" and "actual"
values as such.

Change-Id: I433d263b873286a54725b7a2a0ba536dd29aae47
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/262664
Reviewed-by: Mayank Patke <fishythefish@google.com>
Commit-Queue: Nicholas Shahan <nshahan@google.com>

tests/modular/issue38703/main.dart

@@ -4,5 +4,5 @@ import 'package:expect/expect.dart';
 const Map<Key, String> m = {someKey: "PASSED"};
 
 main() {
-  Expect.equals(m[someKey], "PASSED");
+  Expect.equals("PASSED", m[someKey]);
 }