"Edit summary" button for the text with summary widget has unthemable markup

In the text.es6.js file there is the following code:

          // Set up the edit/hide summary link.
          const $link = $(
            `<span class="field-edit-link"> (<button type="button" class="link link-edit-summary">${Drupal.t(
              'Hide summary',
            )}</button>)</span>`,
          );

That is outputting some markup without using Drupal.theme().

That markup should be wrapped with a theme function so themes can override it.