Parser cannot extend acornDynamicImport, cause it's not a function

I run this source code and the result is incorrect

```javascript
const parser = Parser.extend(classFields, acornDynamicImport);
```

result:
```
...\node_modules\acorn\dist\acorn.js:571
Parser.extend = function extend () {
var plugins = [], len = arguments.length;
while ( len-- ) plugins[ len ] = arguments[ len ];

var cls = this;
for (var i = 0; i < plugins.length; i++) { cls = plugins[i](cls); }
^

TypeError: plugins[i] is not a function
```

Reproduce the issue with the shown Parser.extend(classFields, acornDynamicImport) snippet and inspect the Parser.extend entry point, using the stack location in node_modules/acorn/dist/acorn.js as the starting point. Check why acornDynamicImport is not being accepted as a plugin; done means the parser no longer raises the TypeError and produces the expected result.

Written by the indexing model from the issue text.