ES6+ code sometimes finds its way into the fallback build, which should only include ES5, when one of our dependencies decides to ship their package with newer syntax.

Calypso added a new build-time check that ensures no non-ES5 syntax is present in the generated fallback build, breaking the build if that's the case. It would be nice to add this to Jetpack as well. This will help us catch these issues as soon as they happen, rather than when an IE11 user reports them.

npx eslint --env=es5 --no-eslintrc --no-ignore ./_inc/build ./_inc/blocks

-- Automattic/wp-calypso#36596