[js] ObjectMap<String, {}> Cannot create property '__id__'

Source code:
```haxe
class Test {
static function main() {
var map = new haxe.ds.ObjectMap();
map.set('abc', {});
trace(map);
}
}
```

Error:
```
Uncaught TypeError: Cannot create property '__id__' on string 'abc',https://try.haxe.org/program/d09d3Dc8/run?r=0.10555269501561204,line:23
```
All Haxe versions

Start by running the provided Test.main reproduction through the linked try.haxe.org example and inspect the generated JavaScript for ObjectMap. Trace why setting the string key attempts to create __id__ on the key, then confirm the corrected behavior by rerunning the example without the TypeError.

Written by the indexing model from the issue text.