Test element type of generator functions

[eernstg]

We've noticed a soundness issue with the element type of generator functions, as mentioned in
dart-lang/language#3218.

@sgrekhov, it would be great if you could star that pull request, and write some language tests when it has been resolved!

In particular, we should cover the currently unsound behavior associated with examples given here, and we should cover the run-time types of objects returned by generator functions. We'd expect is Iterable<T> for a suitable T, or is Stream<T>. It's a bit less convenient to verify that the iterable/stream has an actual type argument which isn't too special (if we expect an Iterable<num> we shouldn't get an Iterable<int>), but it should be possible to build on this idea:

X combine<X>(X x, X _) => x;

void main() {
  List<num> xs = <int>[1];
  print('Combine for int:');
  (xs.reduce as dynamic)(combine<int>); // Type correct at run time. Ignore the type checker.
  print('Still alive. Combine for num:');
  xs.reduce(combine<num>);
  print('Not reached');
}