useBuiltIns uses versions of the helpers that do not import even
internal polyfills from core-js.
useESModules uses versions of the helpers that do not go through
transform-es2015-modules-commonjs. This allows for smaller builds in
module systems like webpack, as it doesn't need to preserve commonjs
semantics.
This includes changes to the babel-runtime build-dist script, which
will build the versions of the runtime helpers to be used by
combinations of useBuiltIns and useESModules.
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