Extend strong mode "impossible cast" warnings to explicit casts

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 .

```dart
void test() {
  int x = (true as int); // Make this an error
}
```

cc @Hixie @lrhn @floitschG @eernstg @munificent 