JavaScript: No way to add Symbol.asyncIterator method to a class

I would expect

```haxe
@:native("[Symbol.asyncIterator]")
public function asyncIterator() {
return this;
}
```

to work, but that puts the literal quotes into the JavaScript which we don't want

No files or tests are named in the issue. Start by tracing JavaScript generation for the @:native annotation and compare the emitted member name with the requested Symbol.asyncIterator form; done means the generated property uses the symbol key without literal quote characters.

Written by the indexing model from the issue text.