Migration: fix trial_migration.dart's handling of required

Now that the migration tool inserts `required` rather than `@required`
for required parameters, we need the trial_migration tool to
understand the new behavior.

Change-Id: I401310930120cf6e57af7ab633983b44415d404f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/119081
Reviewed-by: Samuel Rawlins <[email protected]>
Commit-Queue: Paul Berry <[email protected]>

Author: stereotype441

pkg/nnbd_migration/tool/trial_migration.dart

@@ -107,7 +107,7 @@ class _Listener implements NullabilityMigrationListener {
     } else if (edit.replacement == "import 'package:meta/meta.dart';\n" &&
         edit.length == 0) {
       ++numMetaImportsAdded;
-    } else if (edit.replacement == '@required ' && edit.length == 0) {
+    } else if (edit.replacement == 'required ' && edit.length == 0) {
       ++numRequiredAnnotationsAdded;
     } else if ((edit.replacement == '/* ' ||
             edit.replacement == ' /*' ||