This is almost certainly the root cause of issues. I just fixed this error and released as ember-source@3.17.1 and ember-source@3.16.5 (for LTS users). I wrote a bit of an explanation of what was going on in the PR:
emberjs:master ← emberjs:update-babel
opened 11:50PM - 21 Mar 20 UTC
Due to the way Ember itself is compiled, we have custom versions of the `@babel/… helpers` that we inline into the IIFE that contains Ember. This is obviously pretty unideal (largely because it forces us to be misaligned with Babel and therefore brittle to reasonable changes that they make).
These helpers have been pretty stable for quite a while, but recently Babel 7.9.0 introduced a couple of new helpers:
* `createSuper` allows subclassing from native classes, even when partially transpiled).
* `createForOfIteratorHelperLoose`
Versions of `ember-source` prior to 3.13 fully transpiled with the Babel version in `emberjs/ember.js`s _own_ `yarn.lock` at publish time. The build process was significantly changed in `ember-source@3.13.0` so that `ember-source` would behave just like a normal addon. This was a **huge** improvement!! Unfortunately, those changes also made us vulnerable to changes made to Babel (new helpers added).
We **must** resolve that issue (likely by removing these custom external helpers in favor of using `includeExternalHelpers` from `ember-cli-babel`), but in the meantime this commit adds the required helpers which should fix the reported issue in supported Ember versions.
Fixes https://github.com/emberjs/ember.js/issues/18827
Can you confirm that you are using ember-source@3.17.1?
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