dynamic import at top level fails

A)
```
async function main() {
const [
React,
{hydrate},
] = await Promise.all([
import('react'),
import('react-dom'),
]);

hydrate(

, document.getElementById('root'));
}

main();
```

fails with an error:
```
Uncaught ReferenceError: exports is not defined
at 0.js:1
(anonymous) @ 0.js:1
app.js:817 Uncaught (in promise) Error: Loading chunk 0 failed.
```

If I remove the Promises, it works:
B)
```
import React from 'react';
import {hydrate} from 'react-dom';
hydrate(

, document.getElementById('root'));
```

Ironically enough, if I replace (B) with (A) now (while meteor is running), the app starts working. It fails again when I restart meteor. Can someone guide me on fixing/debugging this?

Reproduce example A in the meteor-webpack project and compare it with example B, including a restart of Meteor to confirm the behavior. Inspect the generated 0.js and app.js around the reported exports error and chunk-loading failure; done means the dynamic import works reliably after restarting Meteor.

Written by the indexing model from the issue text.