type inference for `b` in ./tests/build/list_slice.dart is wrong, it should be `int[]`
in 

https://github.com/py2many/py2many/blob/d2118d40aa065bd944208ca31a9fc2f9b29a563f/tests/test_unsupported.py#L70

```
    "list_slice": "a = [1, 2]; b = a[0:1]",
```

If we run this in Python:
```
>>> a = [1, 2]; b = a[0:1]
>>> b
[1]
>>> type(b)
<class 'list'>
```

the correct type inference for `b` should be `int[]`.

However, current Dart output:
```
$ cat ./tests/build/list_slice.dart 
 main(List<String> argv) {
final List<int> a = [1, 2];
final int b = (a[0..1] ?? (throw Exception("key not found")));}
```

current D output:

```
$ cat ./tests/build/list_slice.d
void main(string[] argv) {
    int[] a = [1, 2];
    int b = a[0 .. 1];
}
```

Both because `b` now is typed as `int`.