Updated Drupal to 8.6. This goes with the following updates because it's possible...

[yaffs-website] / web / core / lib / Drupal / Component / Diff / composer.json

{
  "name": "drupal/core-diff",
  "description": "Diff.",
  "keywords": ["drupal"],
  "homepage": "https://www.drupal.org/project/drupal",
  "license": "GPL-2.0-or-later",
  "require": {
    "php": ">=5.5.9",
    "symfony/polyfill-mbstring": "~1.0"
  },
  "autoload": {
    "psr-4": {
      "Drupal\\Component\\Diff\\": ""
    }
  }
}