Specifying Babel options results in 'import' in the middle of bundle

Error:
> Build Error (TerserWriter)
>
> "Import" statement may only appear at the top level
>
> Stack Trace and Error Report: [/tmp/error.dump.212d6b5aecc3b277dd35bac22e8187ec.log](https://github.com/babel/ember-cli-babel/files/6204971/error.dump.3c3bfde42fe46ed0da35e4aaa4b458b0.log)

Steps to reproduce:
```bash
npm install -g ember-cli
ember new testapp --lang en
cd testapp/

ember install ember-cli-babel

cat <<'EOF' > ember-cli-build.js
'use strict';
const EmberApp = require('ember-cli/lib/broccoli/ember-app');
module.exports = function (defaults) {
let app = new EmberApp(defaults, {
'babel': {
useBuiltIns: 'usage',
corejs: {
version: '2.6.12',
},
},
});
return app.toTree();
};
EOF

npm run build
```