Analyzer not type checking elements in `for in` loops.

The following program is accepted by the analyzer but rejected by the front end:
```dart
f(List<String> strings) {
  for (int i in strings) { // BUG
    print(i);
  }
}

main() {
  f([]);
}
```
I'm pretty sure the front end behavior is correct and the analyzer is missing a type check here.