Invalid "is" operator

The following should fail at compile time :

```haxe
"" is Int
```

`is`operator should only compile is there is a possibility for it to be true.

The issue names no files, tests, or entry points. Reproduce compilation of `"" is Int`, then locate the compiler path handling the `is` operator; done means this expression fails at compile time while valid `is` uses remain accepted.

Written by the indexing model from the issue text.