Emit default case for exhaustive switches on JS backend

[lucaswerkmeister]

The JVM backend always emits this default case if a switch does not have an explicit else case:

else throw new .com.redhat.ceylon.compiler.java.language.EnumeratedTypeError("Supposedly exhaustive switch was not exhaustive");

I think it would be a good idea to do this on the JS backend as well. After all, the JS platform makes even less guarantees at runtime than the JVM (e. g. no guarantee that a String member actually contains a String value), and so a switch might not be exhaustive not only due to compiler bugs (e. g. #6311), but also simply because other parts of the program might not adhere to Ceylon’s typing rules (for example, TypeScript enums are just glorified integers, and there’s nothing to prevent someone from assigning 1337 to an enum with only the cases 0 and 1).

[jvasileff]

This should be a bug IMO, given the language guarantee and the type of bugs that may result when it is not met.

[chochos]

The JS backend already adds this check when dealing with dynamic types.

[chochos]

However, if you do this, with no dynamic blocks:

String|Integer foo() => "It's a string";

shared void run() {
    switch(x=foo())
    case (is Integer) {
      print("It's an int");
    }
    case (is String) {
      print("It's a string");
    }
    print("realmente fue ``className(foo())``");
}

And then override foo with a native file ``function foo() { return 3.14; }` then the switch won't cover that case and it's going to be hell to debug. So I guess this might be necessary after all.

[lucaswerkmeister]

Thanks!