Migration: allow clients to use CommandLineOptions.

Previously this class was marked `@visibleForTesting`, but we need
clients to be able to create instances of this class in order to drive
the migration process.

Change-Id: Ibb8a4cda342942dcb15e6fd7625387d246b41f84
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/153800
Reviewed-by: Samuel Rawlins <srawlins@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>

Author: stereotype441

pkg/nnbd_migration/lib/migration_cli.dart

@@ -44,7 +44,6 @@ String _removePeriod(String value) {
 
 /// Data structure recording command-line options for the migration tool that
 /// have been passed in by the client.
-@visibleForTesting
 class CommandLineOptions {
   static const applyChangesFlag = 'apply-changes';
   static const helpFlag = 'help';