An example of my proposal to ban/lint "covariant Function"

ban_covariant_function.dart

void main() {
  // [1] Will fail at runtime.
  var cell1 = new Cell1();
  try {
    cell1.registerCallback((String foo) {});
  } on TypeError {
    print('cell1.registerCallback() failed');
  }
  
  // [2] Will fail statically.
  var cell2 = new Cell2();
  
  // ERROR: (String) => Null can't be assigned to (dynamic) => void.
  cell2.registerCallback((String foo) {});
  
  // [3] User intended (hardest to remember to do).
  var cell3 = new Cell3();
  cell3.registerCallback((String foo) {});
}

typedef void CellValueChanged<T>(T value);

class Cell1 {
  CellValueChanged _valueChanged;
  void registerCallback(covariant Function callback) {
    _valueChanged = callback;
    _valueChanged(null);
  }
}

class Cell2 {
  CellValueChanged _valueChanged;
  
  void registerCallback(void Function(dynamic) callback) {
    _valueChanged = callback;
    _valueChanged(null);
  }
}

class Cell3 {
  CellValueChanged _valueChanged;
  
  void registerCallback<V>(void Function(V) callback) {
  	_valueChanged = (dynamic value) => callback(value);  
  }
}