SyntaxError: Unexpected keyword 'default'

## What's the issue?

The build process crashed with a message:

```
node_modules/.pnpm/acorn@7.3.1/node_modules/acorn/dist/acorn.js:2927
var err = new SyntaxError(message);
^
SyntaxError: Unexpected keyword 'default' (18:16)
```

## How do we reproduce the issue?

```js
// rollup.config.js
import rollupResolve from '@rollup/plugin-node-resolve';
import rollupCommonjs from '@rollup/plugin-commonjs';
import rollupReplace from '@rollup/plugin-replace';
import rollupPostcss from 'rollup-plugin-postcss';
import rollupJson from '@rollup/plugin-json';
import rollupClosureCompiler from '@ampproject/rollup-plugin-closure-compiler';

export default {
input: 'src/index.js',
external: Array.from(external),
plugins: [
rollupResolve({
browser: true,
}),
rollupCommonjs(),
rollupPostcss({
extract: new URL(`./dist/${libName}.css`, import.meta.url).pathname,
minimize: true,
}),
rollupReplace({
values: {
'process.env.NODE_ENV': JSON.stringify(
isProduction ? 'production' : 'development'
),
},
preventAssignment: true,
}),
rollupJson(),
rollupClosureCompiler(),
],
output: {
format: 'es',
file: 'dist/lib.js',
},
};
```

And in `src/index.js`, there is something like this: `export default foobar;`

If I remove the rollup-plugin-closure-compiler plugin, I could run rollup build successfully.

Start by running the supplied rollup.config.js reproduction with src/index.js exporting a default value and @ampproject/rollup-plugin-closure-compiler enabled. Compare the build with that plugin removed and confirm that the error occurs only with it present; done means the same build succeeds while retaining the plugin and default export.

Written by the indexing model from the issue text.