Attaching a JS library to every page
It looks like the javascript file WERE included before, but they were in the footer. When jQuery code is placed in a basic page, that appears to not work. However, forcing the js files in the header allows everything to run.

I changed my module file to be:

&lt;?php
function flot_d8_page_attachments(array &amp;$page) {

 $page['#attached']['html_head'][] = [
    // The data.
    [
      // The HTML tag to add, in this case a &lt;script&gt; tag.
      '#tag' =&gt; 'script',
      '#attributes' =&gt; array(
        'language' =&gt; "javascript",
        'type' =&gt; "text/javascript",
        'src' =&gt; "http://test.mysite.com/core/assets/vendor/jquery/jquery.js",
      ),
      '#value' =&gt; '',
    ],
    // A key, to make it possible to recognize this HTML &lt;HEAD&gt; element when altering.
    'jquery-define'
  ];

  // Attach the flot asset on every page.
  $page['#attached']['html_head'][] = [
    // The data.
    [
      // The HTML tag to add, in this case a &lt;script&gt; tag.
      '#tag' =&gt; 'script',
      '#attributes' =&gt; array(
        'language' =&gt; "javascript",
        'type' =&gt; "text/javascript",
        'src' =&gt; "http://test.mysite.com/modules/flot_d8/flot/jquery.flot.js",
      ),
      '#value' =&gt; '',
    ],
    // A key, to make it possible to recognize this HTML &lt;HEAD&gt; element when altering.
    'flot-define'
  ];

}


And my Basic-Page javascript now works.