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While this could be removed, I am a bit worried that it will break users using @babel/register @babel/preset-env with corejs: 3 without explicitly listing it in their dependencies.

cc @babel/babel

Perhaps we should move the core-js dep to preset-env, then?

Perhaps we should move the core-js dep to preset-env, then?

preset-env doesn't rely on core-js. Only it's output, when explicitly enabled by the user, does.
Also, we would need somehow to add both core-js 2 and 3 without aliasing them so it's not possible.

this error will surface their broken package.json most efficiently, at build time

Actually, @babel/runtime is executed at runtime by definition 😛
And also if it wasn't, the error would be a MODULE_NOT_FOUND thrown at runtime because of the injected core-js imports.

ok, fair point on that. still tho, theoretically any test of any kind on a babelified file would expose it?