test.dart

void startNewGame() {
  myCells.forEach((cellElement) => cellElement.setInnerHtml("a"));
}

// with function

void startNewGame() {
  myCells.forEach(resetElement);
}

void resetElement(Element cellElement) {
  cellElement.setInnerHtml("a");
}

// the same, in different way

void startNewGame() {
  for (var cellElement in myCells) {
    cellElement.setInnerHtml("a");
  }
}