make js.array accept iterables

js.array accepts only List, but maybe we want it to accept any iterable?

This will make it easy to then use common iterable APIs like 'mappedBy' and 'where' before wrapping something into a js.array. For example:

```dart
var myList = [1,2,3,4,5];
js.array(myList.where((x) => x > 3))
```

instead of:
```dart
var myList = [1,2,3,4,5];
js.array(myList.where((x) => x > 3).toList())
```