Should eliminating implicit casts include casts during iteration.

Currently in Dart (or soon in the case of the spread), the following is a legal program:

```dart
main() {
  var a = <Object>[1, 2, 3];
  for(int x in a) {
    print(x);
  }
  <int>[...a];
}
```

There are implied casts on each element of the iteration, and on each element addition from the spread.

If we eliminate implicit casts, should we include these as well?
