Private property transpilation broken w/ externalHelpers enabled

[nickschot]

ember-cli-babel currently has a pinned dependency of @babel/runtime 7.12.18. If a newer version (like 7.17.9) for whatever reason ends up on the root level of node_modules (like other dependencies bringing in a newer version) it may break the build at runtime in certain cases. The specific case that I ran into occurs when using private properties in native classes like in this reproduction: https://github.com/nickschot/babel-test-app/blob/master/app/controllers/application.js#L9

with externalHelpers: true configured in ember-cli-build.

When running the app this will result in a runtime error like:

The reproduction uses a private property in an app controller, but the same issue happens when addons use private properties in for example components.

Note that for the linked reproduction I've added a dependency on @babel/runtime 7.17.9 to force this issue. Disabling externalHelpers and/or making sure the version of @babel/runtime in the root of node_modules is 7.12.18 prevents the issue from occurring.

The pinning happened here: #385
May be somewhat related to: #420