ERROR in http.min.js from Terser

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When I set webpack config `mode: 'production'`，it throw error:

```
ERROR in http.min.js from Terser
"Export" statement may only appear at the top level [http.min.js:1161,0]
```

But when I set `mode: 'development'`，it works...I can't figure out why.

```js
// http.js

const http = () => {}

http.version = '__version__';

export default http;
```

```js
// webpack.build-lib.js
const path = require('path');
const ReplacePlugin = require('../lib/webpack-plugin-replace');
const rootPath = path.resolve(__dirname, '..');
const pkg = require('../package.json');

const libName = 'http';
const entryFile = path.resolve(rootPath, `src/${libName}.js`);

module.exports = {
entry: entryFile,
mode: 'production',
output: {
path: path.resolve(rootPath, 'lib'),
filename: `${libName}.min.js`,
libraryTarget: 'umd2',
libraryExport: 'default',
},
plugins: [
new ReplacePlugin({
exclude: [
/node_modules/,
],
patterns: [{
regex: /__version__/g,
value: pkg.version,
}],
}),
],
};

```

Start with http.js and webpack.build-lib.js, then run the production build that uses mode: 'production' and Terser; compare it with the development build. Trace how the ReplacePlugin output reaches lib/http.min.js and determine why the export is no longer at the top level. Done means the production build completes without the reported Terser error.

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