public function update_9001000() {
    $this->updater->syncWithTemplate('.gitignore', TRUE);
    $messages = ['.gitignore has been updated. Review it for any custom changes that may have been overwritten.'];

    $formattedBlock = $this->updater->getFormatter()->formatBlock($messages, 'ice');
    $this->updater->getOutput()->writeln("");
    $this->updater->getOutput()->writeln($formattedBlock);
    $this->updater->getOutput()->writeln("");

    $this->updater->syncWithTemplate('blt/ci.blt.yml', TRUE);
    $messages = ['blt/ci.blt.yml has been updated. Review it for any custom changes that may have been overwritten.'];

    $formattedBlock = $this->updater->getFormatter()->formatBlock($messages, 'ice');
    $this->updater->getOutput()->writeln("");
    $this->updater->getOutput()->writeln($formattedBlock);
    $this->updater->getOutput()->writeln("");

    // Update composer.json to include new BLT required/suggested files.
    // Pulls in wikimedia/composer-merge-plugin and composer/installers settings.
    $project_composer_json = $this->updater->getRepoRoot() . '/composer.json';
    $template_composer_json = $this->updater->getBltRoot() . '/subtree-splits/blt-project/composer.json';
    $munged_json = ComposerMunge::mungeFiles($project_composer_json, $template_composer_json);
    $bytes = file_put_contents($project_composer_json, $munged_json);
    if (!$bytes) {
      $messages = ["Could not update $project_composer_json."];
    }
    else {
      $messages = ["Updated $project_composer_json. Review changes, then re-run composer update."];
    }

    $formattedBlock = $this->updater->getFormatter()->formatBlock($messages, 'ice');
    $this->updater->getOutput()->writeln("");
    $this->updater->getOutput()->writeln($formattedBlock);
    $this->updater->getOutput()->writeln("");
  }