Extend strong mode "impossible cast" warnings to explicit casts

[leafpetersen]

Strong mode issues an error if a literal or an instance creation expression is implicitly cast to an incompatible type, since such a cast cannot possibly succeed in strong mode. We should consider issuing the same error on explicit casts .

void test() {
  int x = (true as int); // Make this an error
}

cc @Hixie @lrhn @floitschG @eernstg @munificent

[lrhn]

Was this done or not?

In general, I think we could improve the experience around explicit casts.
For example [] as List<int> could infer the <int> (since interface instantiations are unique for a class, we known that it must be <int>).

[srawlins]

This is definitely not done in analyzer.