An EitherType constraint causes oddities when used as a map key

Sample:
```haxe
import haxe.extern.EitherType;

@:keep class Test {
static function main() {
var tc = new Impl();
trace(tc.labels[One]);
}
}
abstract class Base> {
public var labels:Map = new Map();
public function new() {}
}
class Impl extends Base {

}
enum ETest {
One;
Two;
}
```
So, two things:
1. Perhaps this should not compile unless there's a multitype that can accept both key types.
2. On JS with `-dce full`, this results in compiler picking StringMap but culling its get() method, causing a runtime error.