Message on PHPUnit requirements problem needs small update

Problem/Motivation

I recently updated a local development site from 8.5.0 to 8.5.1. The site was pretty much just Drupal Core, and I had the Testing (simpletest) module enabled.

I did the update by downloading the .tar.gz tarball from drupal.org, and replacing the core, vendor, and top-level files from my existing installation with the new files. This included the composer.json and composer.lock files.

Then I went to update.php to run database updates. I saw a message saying:

Requirements problem

PHPUnit dependency

The testing framework requires the PHPUnit package. Please run 'composer install --dev' to ensure it is present.

So I went to the command line and ran

composer install --dev

This worked fine, but it gave me a message saying:

You are using the deprecated option "dev". Dev packages are installed by default now.

So, it seems like the requirements message needs an update to remove the --dev option from the suggested command.

Proposed resolution

Find out where this message is being generated that says to run composer with --dev. Remove the --dev option from the command.

Remaining tasks

Make a patch.

User interface changes

The message telling you to run composer install will not tell you that you need to use the --dev option.

API changes

None.

Data model changes

None.