Some issue with resolution/variance for abstracts in Map key parameter

Example:
```haxe
abstract Foo({ bar: Int }) {}

function main() {
final map: Map = new Map(); // error: Foo should be { bar : Int }
}
```

There is no error If underlying type for `Foo` is simply `Int`.
There is no error If `Foo` has direct `to` or `@:forward.variance`.

Looks like during selection of implementation for `Map` key type was replaced by underlying one.