Constant list/map is modifiable

The following does not throw in DDC.

```dart
void main() {
  List<String> list = const [];
  list.add("hello");
  print(list);
}
```

On the VM, it generates:
```
Unhandled exception:
Unsupported operation: Cannot add to an unmodifiable list
```

EDIT(jmesserly): Maps need to be fixed as well