No way to verify that a list argument's entries are valid in a const assert

```dart
abstract class MultiChildRenderObjectWidget extends RenderObjectWidget {
  const MultiChildRenderObjectWidget({ Key key, this.children: const <Widget>[] })
    : assert(children != null),
      assert(!children.any((Widget child) => child == null)), // <-- this line won't compile
      super(key: key);
  // ...
}
```