Problem solved! It turned out that my old `index.html contained
<script src="https://unpkg.com/@webcomponents/webcomponentsjs@2.0.3/custom-elements-es5-adapter.js"></script>
which was not present in the starter project with Angular 8. Instead it seems that building with Angular 8 automatically injects
<script src="runtime-es2015.js" type="module"></script><script src="runtime-es5.js" nomodule defer></script><script src="polyfills-es5.js" nomodule defer></script><script src="polyfills-es2015.js" type="module"></script><script src="styles-es2015.js" type="module"></script><script src="styles-es5.js" nomodule defer></script><script src="scripts.js" defer></script><script src="vendor-es2015.js" type="module"></script><script src="vendor-es5.js" nomodule defer></script><script src="main-es2015.js" type="module"></script><script src="main-es5.js" nomodule defer></script></body>
in the file. Removing the existing reference to custom-elements-es5-adapter.js solved the problem.